First question as an agony aunt

From the lovely, Anonymous, I am going to assume you're a girl, though my response would probably be the same if you were a guy.
Anonymous says-

"There is a boy so pretty I cannot bear. All day I wish to kiss him and it is difficult but I must not.
We are friends and I do not want it to be spoiled. What can I say...Do I stay quiet forever?"

The short answer is this, No.
The more in-depth answer is this, I understand the whole not wanting to spoil the friendship thing, but imagine how much more worth it the risk is, if you kiss him one day and it all ends with a happily ever after.
Now imagine you never take the chance, have a string of boyfriends until you eventually just settle for someone in your 30's who makes you happy, not ecstatic, not thrilled, not filled with passion every time the guy you settled for even smiles at you. But happy all the same. You'll never know if this guy, the one you want to kiss, would thrill you and make you happier than you ever thought.

Of course it could all go the other way, you could tell the boy that you want to kiss him, and he might knock you back, or he might say he feels the same but after a while it doesn't work out. This sounds bad, but isn't it worth the risk?

Anyway, I'll restrain myself on the rambling and get back to the question. Is the reason you must not kiss him just that you don't want to spoil the friendship? If that's all that's stopping you, its really not so bad.
I think the only reasons you shouldn't kiss this boy are if -

-You have a boyfriend
-He has a girlfriend
-You're related

If none of the above apply, then girl, you're good to go. (If any of them do apply, then whoa whoa whoa, let me know and I'll dish out more advice)

So assuming all systems are go, lets cover the kissing. I understand this part of your predicament very well, kissing is my number one favourite thing in the whole world. However, unless you think he feels the same, its probably not the best idea to just ambush him with kisses (as much as I reaaaaally do want to advise you to do that).
This brings me to the next bit. Do you think he feels the same? Do you think there is even a maybe?

That opens up a whole other bag of, stuff. Its hard working out if a boy you like likes you back. Its been years since I've really been in that sort of situation, and only with the gift of hindsight have I picked up the signals that I naively missed the first time around. I think its the same for everyone. You just don't realize what what he's saying actually means. I think boys get accused of doing it all the time, missing what us girls think are totally strong signals, but in my old age I realize I've missed fucking loads, not realising what was going on. I guess its just as hard for everyone.
I think basically you've gotta trust your instinct, your gut feeling, your intuition. If you think its looking like he's into you, then its quite likely that he is.

In conclusion, don't stay quiet forever. Tell him you like him, and that if he wouldn't mind, you want to kiss him please. If you're exceptionally nervous, maybe give it a week or so. Or if you're too nervous to tell him to his face, maybe you could leave a little note for him. It sounds like its your time to do it, good luck!
Let me know how you get on, and if you're crazy enough to want any more advice, agony Bianca will be here!