Ones relationship with others helps define who they are because relationships are a huge factor in how we view ourselves. If someone had a bad, unhealthy relationship with a person they might view themselves badly and therefore change who they are. This also means that a good relationship can make someone happy and view them self spork let. Relationships with others also influence the things a person might do in there daily life, since they might want to do the same thing as the other person in order to remain in that relationship.
Hope things makes sense and helps! :)