Comparison level for alternatives falls under the Social Exchange Theory of Attraction. It is the idea that people are satisfied in their relationships based on their extent to which they perceive their ability to exchange their current relationship for a better one.
I had a friend in high school that was very unsatisfied with her boyfriends. She had one boyfriend that she didn’t really like, but kept dating him. One day we had a long conversation about it. She said the reason she kept dating this boy was because she didn’t think she could find anyone better at the time. She didn’t really like him she just didn’t have anyone better at the moment. However, she believed that she could get a better relationships and could drop him whenever she wanted.
This is an example of comparison level for alternatives because she based her satisfaction for her relationship/friendship on her perceived ability to find a better relationship. She eventually did find a “better” relationship and eventually dropped the guys she was dating. Now that she is married and she believes she can’t get a better guy, she feels much more satisfied and secure in her relationships.