Joining with another person, other people, or a group can represent an actual or desired:
Commitment to the other person or people (such as joining in marriage, joining a community)
Commitment to a shared cause or goal (such as joining a union or philanthropic organization)
Acceptance of a creed, set of beliefs or intentions, etc. (such as joining a religion)
Interaction with others who share your interests
Feeling of belonging or acceptance by others
Feeling that you're part of a community or whole