Well, first of all, I'd want to live with her, in this life, for a few years before doing it... I wouldn't want to do it immediately, or anything. The reason to me wanting to do it, and why I am looking for a girl who agrees on it, is because I would not want either of us to grow old, as I value beauty too much, but then there is also the fact that like it or not, both she and I have human minds, and the human mind has limitations that cause one to get used to stuff, finding them less exciting, and that sadly includes love. So we'd have to die early on, so our human minds would not corrupt our love for each other, making it less exciting, or even making it die out, with time. True love is supposed to never end, so by joining in a romantic suicide pact, we would happily continue experiencing our fully exciting love, in "death", for eternity, where we are rid of our flawed, human bodies.
It wouldn't be a painful way to go. There are painless, non-violent ways to die, as well.
Flowers are nice, too. I'd like to cook food for her with roses, one day. They do that in China... it's supposed to be very tasty.