Insecurities & socialization

Insecurities cause problems with socialization, I find that some people need to take a close look at how this can create problems for not only themselves but others. It was made really clear to me this past weekend. While attending a barbecue, a female created a big scene and embarrassment for her boyfriend as well as herself, due to her own insecurities. Trying to understand the situation, I began to observed this female very closely and reflect back on her interaction with others before the incident (where she began to argue with and fight on her boyfriend). I recalled that she appeared to have a attitude and had no socialization before or after the incident with females. I later found that her anger was due to jealousy when her boyfriend interacted with other females; as long he was talking with males she was ok, but when it came to females she would become extremely angry. But why? This man was with her and made it clear to everyone that this was his significant other. I feel that if a person is disrespectful enough to persue another relationship in the presence of their husband, wife, or significant other, then WHY would you be with them! Why is it that she felt so threatented? Was she insecure in the relationship, did she have low self esteem, was she jealous of attractive woman, or was it she didn't know how to socialize? Whatever the case, her insecurities caused for others
to feel uncomfortable to say anything to her or him. Her actions made her boyfriend appear embarrassed, angry, and anti-sociable (due to her actions he dared to move out of her sight). Im almost sure it caused a problem for their relationship and his relationships with others as well. What is it that causes a person to be so insecure with themselves that because they cannot socialize they don't want to see their husbands, wives, or significant others, socialize either?