Why does it seem like some people are always in a relationship?

Everyone's different: They may fear being alone, or they are unsure of what they want out of a relationship so they break up & move on to the next person. It is important for one to love himself before he can love someone else. It's healthy to look inside oneself and determine if all of this dating is a way to stay happy, and if you're a fundamentally unhappy person on your own. Take time to figure things out.

Maybe they are: I imagine you're referring to romantic relationships? Some people just don't like being alone. Actually, we're all "in relationship" all the time, even if not in the outer world. We all have holograms of important others inside us, where we continue to relate to them. We can even mistake new people for the images we carry -- a sure path to disappointment. Best to let the new person be new!