Relationships between people aren't perfect and things happen, sex is a very healthy way of mending the fences and hitting a retart button of sorts. Sex is also very good for stress release, which as parents of a young child we get a lot of built up in us. I'm noticeably more irritable if it's been more than a few days without us finding some "couple time", so it's not really just a matter of wanting it, it does make things easier.
Sex is a big desire for most people. It is one of the things that encourages us to meet people, it helps ease depression, and makes people happy. For some it results in children
and populates the planet. It is part of an emotional and romantic bond with your partner as well.