Pain v. Pleasure

I was looking back through Phaedo, and I came across the part where Socrates says that whenever one experiences one thing, it's opposite is soon to follow:
"What a strange thing that which men call pleasure seems to be, and how astonishing the relation it has with that is thought to be its opposite, namely pain! A man cannot have both at the same time. Yet if he pursues and catches the one, he is almost always bound to catch the other also, like two creatures with one head."
I think that that is an interesting view on life, especially because I've known a lot of people that have subscribed to it. I think, as an observation, it is insightful and often true, but when it becomes a view on life it can have problems. I say this, because I can think of people in the past for whom it has become a self-for filling prophecy. When things are good, they think something bad will happen and act accordingly, and then something bad does happen, most likely because of their state of mind, (except when it has nothing to do with their control of course). But then I started thinking; could it be that the state of mind is all a part of the cycle and that it is a part of the good event leading to the bad event, or the bad event leading to the good event?
Despite the universal moments of doubt, I tend to think that we have more control over our lives than we often think. Sometimes, the belief that if you experience pleasure, pain is not far off, and vis versa, is more destructive than anything.