People smoke marijuana out of pleasure, that's pretty much it. I know that most are generally introduced to it out of peer pressure, but they don't stick at it because they developed some sort of addiction, nor do they experience physical withdrawal upon quitting. It's just a matter of endlessly experiencing that "high" feeling. To answer your question, no. I didn't need any rehab for quitting marijuana, I simply quit and never looked back. That's probably the best thing about marijuana: you can quit whenever you want to, haha.
I have heard that some places have marijuana rehab centers but it is very hard to find. Usually weed does not have any physical symptoms during the withdrawal period that are harmful/deadly in any way, so rehab centers are not really emphasized. Things like alcohol withdrawal have very heavy and negative side effects so more places emphasize the need to have rehab centers. I can imagine in the future that there will be more and more weed rehab centers open just how there are rehab centers for sex addicts!