I'm not a real scientist. I don't even play one on TV. But unlike most real scientists, I have the benefit of not being blinded by rigidity, callousness and arrogant logic regarding other living things. What I'm saying is that I firmly believe that all animals, even slugs and snails, are capable of emotion, suffering and pleasure. After all, are fear, stress, curiosity and anger not emotions? It's quite clear that animals feel these things, so why couldn't they experience the complete package? To anyone who pays attention, it should be obvious that they do.
If they can feel, then they can certainly also act on those feelings. I've seen my slugs and snails behave toward one another, even ones outside their species, with a moving amount of tenderness and affection. I've also watched them decide to behave like little assholes out of the blue and for no apparent reason, just like people can. Case in point:
On a number of occasions now, I've witnessed altercations between individual slugs that were completely unprovoked. Much of the time, the unfortunate victim was sleeping peacefully, minding his own business, when another slug would be passing by. The passer-by would suddenly notice the slumberer, move toward him and begin prodding him with his lower pair of tentacles. It all looks very innocent until the slug doing the prodding rears back slightly and lunges mouth-first at the vulnerable body of the unsuspecting one who, at this point, has been very rudely awakened and is thrashing around rather violently.
It doesn't always stop there. The aggressor would sometimes give chase (with shocking speed, might I add) and continue biting the other. When this happens, I intervene to prevent anyone from being injured. However, I'm certain that these things go on all of the time when I'm not watching them. Since I don't find the container littered with the mauled remains of dead slugs and snails (nor am I noticeably missing any), these altercations must resolve themselves uneventfully most of the time and are simply a part of their natural social behavior.