Is there really a difference between pain and pleasure?

They technically use the same receptors (nerves), but they do get separated after that. Yes, our attitudes can, to some extent, regulate the amount of pleasure or pain we experience. But biologically, pleasure does motivate us (humans as well as any other animal with a limbic system) and pain does deter us. Pain is important in protecting us; if something hurts we naturally will stop (or want to stop) doing it because if we continue, we will do more harm to our bodies. So, for example, sex might feel good because it helps us to procreate and/or be more intimate with others (which is useful for survival) and putting your hand on the hot stove will cause pain which will hopefully drive you to remove your hand from it to prevent more damage. That's kinda basic and there's obviously exceptions and such but hopefully that helps answer your question.

Funny. I've had my teeth pulled and I've had an orgasm. Feels completely different to me.

You should summon the Cenobites and ask them. They will help you understand... Bwahahahahahahaha!