Life is complicated.
We simplify it as much as we can but all we really do is arrange factors to a more favorable manner of handling complications. Relations with other human beings only adds up to a complicated mess of emotions, obligations, commitments and interactions.
Given that, why bother with a relationship at all?
Whether it is with a lover, a friend, blood relation or casual acquaintance, we seek such relations despite the added value of complicatedness due to the fact that we are social beings. We yearn for contact and connection. Whether it is physical or emotional, it hardly matters. It still gets a lot more complicated.
While living a life is complicated, it is through sharing that life - the experiences, beliefs and emotions and its totality - that we ultimately have a clear grasp on the realities we face. For in those complicated relations, we, at the very least, need not face them alone.