I appreciate the generous impulse. And it's not that I don't like money. But I have a couple reasons for not accepting tips. One is that it doesn't feel entirely professional to me. Another is that it seems a little dubious ethically to take money from someone when they're (hopefully, anyway) floating on a nice endorphin-high.
But the main reason is quite self-interested. People tend to tip too much, and then to feel that they can't come back unless the tip what they tipped before, and they end up pricing themselves out of their own market. I know I did that, when I was a massage client. I'm delighted that you want to tip me: but channel that impulse into getting massage more frequently, and we'll both be happier.