I was fit like you (see my pics for the before afters), and like you, was convinced by my partner to start gaining. Here's what I like about it...
First of all, soft flesh feels better. It feels sensual, not only to the touch (ie: someone else's fat) but the sensation of being touched. I suspect there's a relationship between hormones & fat, because as I've gotten fatter my sex drive & lust has grown.
The act of being fed, and gaining, with a partner becomes a shared, very personal experience. And as you see their lust grow, it will become a cycle of incentive that will push you forward.
There's something amazing about physical transformation - I suspect it's similar for people who lose a lot of weight - in that feeling your body change (in my case, filling up more space, getting bigger, and as I got really big, restricting some movements) is just an incredible journey. When you separate it from the societal pressures to be thin (ie: getting thinner is good, getting fatter is bad), and just boil it down to what it physically is, it is a fascinating and incredible experience. I wouldn't trade it for anything!
I could probably go on and on, but suffice to say: it's a journey I took and recommend.
My only additional advice is to try and be healthy about it (you can get really fat without going to McDonald's every day!).