You're my friend, or perhaps a casual acquaintance. Or maybe you've just seen me at a bar or something. Whichever, something's gone awry in your brain chemistry, and now your feelings toward me are a little more than friendly.
Do not ask me out. Do not offer me a drink. Do not tell me you have a “thing” for me, as though telling me will somehow absolve you of the guilt you feel over mentally straying from your SO. Just don't.
I will not acquiesce to your advances. I am, with all the best will in the world, not interested. I do not need this information.
Prior to your outright telling me, I was not aware of your feelings for me. I have not picked up on your social cues. I cannot read the signs (and if I could I wouldn't believe them).
Your telling me changes things. Whatever stage of friendship we were at, whatever good thing we had going on, you ruined it. I value our friendship/acquaintanceship/strangership and I don't want it ruined by an aberration in your brain chemistry.
Repress those feelings. Squash them down inside. Turn it off like a light-switch. Your culture may tell you that it's healthy to get these things out in the open, but I'm telling you: if you just suppress it hard enough, it will go away. Or, you know, it won't; but either way I won't have to deal with it, and that's what matters.
In all seriousness, I'm frustrated by the concept that a close friendship will or must evolve into a romantic relationship. I think it's because our culture confers ultimacy on romantic/sexual relationships, and this necessarily entails devaluing friendship: think how many movies and TV shows you've seen where the protagonist throws over career, friends, and everything else for the sake of a romantic partner.
I wear flannel and dungarees and am pretty openly gay; and yet, even in this queer progressive community, my very close relationship with a male friend is constantly read as romantic (it honestly couldn't be less so). Thanks, wider culture.
In high school, I categorized my strong feelings for a (male, gay) friend as romantic, even though I had zero physical designs on him, because I'd internalized the cultural messages that any strong feelings I had for a boy must be construed as romantic. I didn't yet have the deconstructive tools to maintain a very intimate but wholly platonic friendship.
Now I do; and now I know that, for me, friendship – not romance, not sex, but real true friendship – is the ultimate in human relationships. And that, in essence, is why I don't want people crushing on me.