Like with most things in life, there is no right answer, and it is only a matter what is right for you.
I enjoy my wife's company and like and respect her as a person. I enjoy the idea that I will hopefully get to grow old with her. The shared experiences in our lives have created a bond that I didn't previously fully appreciate existed. We have influenced each other in very positive ways over time. We have supported each other when life threw us curveballs.
That's all without even getting to the fact that we have kids. My kids absolutely wear me out (more mentally and emotionally than physically), but they wear me out in the way a marathon wears me out. You are exhausted and maybe a little bit broken, but you appreciate even the fatiguing aspect of the experience with some relish. And the fact that they wear me out is a small price to pay for how good it feels to simply love them and be loved by them, for how they look to me, for how it will feel for them to grow and mature to the point where they no longer need me and rebel against me. Watching them develop and interact with their world has been incredibly rewarding.
Before marriage, there was the excitement of the chase, of the new, of the uncertainty. I loved falling head over heels for someone, even if it didn't always turn into a relationship. If my wife left me tomorrow, I don't think I would get married again since I don't intend to have more kids; I would simply engage in serial monogamy in bouts of 3 to 6 months at a time.
I have friends who don't have kids and they seem to be happy and fulfilled and I am often jealous of the freedoms that they have. I can honestly say that I wouldn't trade lives with them, but that doesn't prevent me from recognizing that there are things that they have that I wish that I could at least have from time to time. The ability to travel, the financial flexibility, etc.
But at the end of the day, we are social beings, and while kids may not be right for everyone, being in a relationship from time to time does seem to be fairly important to the happiness of the vast, vast majority of people with whom I have interacted.