At the request of Meliai and Jessica 
I'm interested in this topic and could write a lot, but I'll try and keep it short-ish and just introduce the topic and give people an idea of what the different styles look like. Mel and Jessica, feel free to add whatever I've left out. This information can be used to view your relationship patterns through a different lens, which may help alleviate them. I think most people lean anxious or avoidant, even if they are fundamentally secure.
The short definition of Attachment Theory:
"Attachment theory is a psychological, evolutionary and ethological theory concerning relationships between humans."
Attachment theory was initially developed in response to an observation about how infants reacted to being separated from their primary caregivers (mother or father). The pattern of care provided by your caregivers, coupled with adult experiences, and genetics, form your attachment style in adulthood. There are 2 types of attachment: Secure, and Insecure. Insecure people can be divided into 2-3 subgroups: Anxious and avoidant. Some people have a mixed style, I'll call it mixed. This is the least common. About 50% of people are securely attached.
Anxious: Anxiously attached adults had caregivers who were unreliable in childhood. Sometimes, they would cry out for help and receive help, other times their caregivers would punish them, ignore them, etc. The main thing is that they weren't able to predict how their behaviors would be taken and therefore were not able to learn a healthy dependence on others. They did not learn that their feelings were valid or would be responded to. In adult relationships, this tends to manifest in "neediness"-these people are always filtering for negative signs. "My partner didn't text me back, they are going to leave me." "They came home an hour late from work, they must be cheating on me." They might seek constant reassurance from partners, because they lack the ability to read people's signs and signals in a way that is healthy and normal. During conflict, anxious people want to solve a problem immediately. They are at risk of entering and staying in bad relationships because they always blame themselves for problems and require a relationship to feel whole. This style is also called "preoccupied" because they are constantly thinking about their partner and the status of their relationship.
Avoidant: Avoidant people had caregivers who never met emotional needs in childhood. These people learned that showing emotion and being vulnerable is a bad thing. They put on an air that they don't need other people, and they deactivate and push others away when they try and get close. Deactivating behaviors might look like telling a partner "I need space" and not contacting them for a few days after an intimate conversation, or constantly finding flaws in a partner in order to keep them at a distance. Avoidant shut down during conflict and want to be alone. They are concerned with seeming perfect and in control of themselves at all times. Avoidant are at risk of dating a lot of people and never finding love because they think something better is always around the corner.
Mixed: Vacillate between both.
Secure: Securely attached people had parents who responded consistently to their emotional and physical needs. They are Comfortable with intimacy. Not prone to jealously. Trust people and recognize their need to depend on others and have others depend on them. Solve conflicts in an emotionally attuned way. Recognizes others needs and can respond appropriately. These people do not find relationships stressful.
Here is an info-graphic that might be helpful:

I'm interested in this topic and could write a lot, but I'll try and keep it short-ish and just introduce the topic and give people an idea of what the different styles look like. Mel and Jessica, feel free to add whatever I've left out. This information can be used to view your relationship patterns through a different lens, which may help alleviate them. I think most people lean anxious or avoidant, even if they are fundamentally secure.
The short definition of Attachment Theory:
"Attachment theory is a psychological, evolutionary and ethological theory concerning relationships between humans."
Attachment theory was initially developed in response to an observation about how infants reacted to being separated from their primary caregivers (mother or father). The pattern of care provided by your caregivers, coupled with adult experiences, and genetics, form your attachment style in adulthood. There are 2 types of attachment: Secure, and Insecure. Insecure people can be divided into 2-3 subgroups: Anxious and avoidant. Some people have a mixed style, I'll call it mixed. This is the least common. About 50% of people are securely attached.
Anxious: Anxiously attached adults had caregivers who were unreliable in childhood. Sometimes, they would cry out for help and receive help, other times their caregivers would punish them, ignore them, etc. The main thing is that they weren't able to predict how their behaviors would be taken and therefore were not able to learn a healthy dependence on others. They did not learn that their feelings were valid or would be responded to. In adult relationships, this tends to manifest in "neediness"-these people are always filtering for negative signs. "My partner didn't text me back, they are going to leave me." "They came home an hour late from work, they must be cheating on me." They might seek constant reassurance from partners, because they lack the ability to read people's signs and signals in a way that is healthy and normal. During conflict, anxious people want to solve a problem immediately. They are at risk of entering and staying in bad relationships because they always blame themselves for problems and require a relationship to feel whole. This style is also called "preoccupied" because they are constantly thinking about their partner and the status of their relationship.
Avoidant: Avoidant people had caregivers who never met emotional needs in childhood. These people learned that showing emotion and being vulnerable is a bad thing. They put on an air that they don't need other people, and they deactivate and push others away when they try and get close. Deactivating behaviors might look like telling a partner "I need space" and not contacting them for a few days after an intimate conversation, or constantly finding flaws in a partner in order to keep them at a distance. Avoidant shut down during conflict and want to be alone. They are concerned with seeming perfect and in control of themselves at all times. Avoidant are at risk of dating a lot of people and never finding love because they think something better is always around the corner.
Mixed: Vacillate between both.
Secure: Securely attached people had parents who responded consistently to their emotional and physical needs. They are Comfortable with intimacy. Not prone to jealously. Trust people and recognize their need to depend on others and have others depend on them. Solve conflicts in an emotionally attuned way. Recognizes others needs and can respond appropriately. These people do not find relationships stressful.
Here is an info-graphic that might be helpful:
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