Because we want to

Change is hard, it is easy to get hung up on petty things. Like, I have always done it that way, or I always keep that there. Living alone for some time confirms us in our habits. It can be a strain learning to accommodate another persons habits. However it can be fun negotiating and compromising so long as we ditch the petty things which don't actually effect our day to day living and concentrate on the bigger things.
It also helps to remember why we invited the other to share our life, for the pleasure of their company. For the way they make us laugh, for the love they give us and help us to feel. For the delight of knowing someone knows us and loves us warts and all.