You can think of it kind of like this.. 要 means to want to, to ask to or will do something. wo yao shui jiao can also means i will go to sleep. 想 means think, would like or want(+sb) to do something.

"I want to..." is a statement of a fact. However, direct speech can sound rude or aggressive in polite or delicate situations, so we use a more indirect form "I'd like to..." to show politeness. To tell the truth, "I'd like to" is far more common when making requests.

This link answers a similar question except "I would like" is used as "I'd like". http://www.italki.com/question/45153 Hope it helps : )