I got a D- in science so bear with me.

Weed releases the "happy" hormone in your brain (I think it's dopamine or something). If you smoke every day (bonus points if you do it routinely - the same time every day), your brain eventually recognizes the action of smoking as the thing it needs to release the happy hormone every day. After a long while of habitually smoking to feel happy, youll eventually depend on it to release dopamine. Kinda like a ghetto prescription.

This can obviously be reversed, but the first few days after you quit will be probably the most sad and depressed you've ever felt. That's because your brain needs time to get used to realeasing dopamine without weed, and it'll take at least a few days before that happens.

I know this is probably not that great of an explanation, but I mainly got it from personal experience. A while back, I used to rely on weed to help me get through a really low point in my life instead of doing the normal thing and just talking to somebody. I'm all better now, but the "withdrawal" was probably worse than the feelings that drew me to it in the first place.