To make them happy, at any and all costs necessary; this is a really hard question, might want to define it a bit.
I've found that usually a list of reasons don't quite justify the intensity of love. Nor can love be boxed into mere words. But, that said, I've fallen in love with my partner because he is my soulmate first and foremost.
Love isn't logical... it's just a feeling. I feel good when I am around the person, they make my day better, we are able to talk about things we want to talk about freely and openly. Of course there is a certain degree of physical attraction as well. So I don't really know if I would say I have a reason. I don't need one. If I feel good with my partner, and my partner feels good with me, and we are able to bond, then that's enough for me.