Can we ever truly act against our own interests?

Think about it, everything we do has some kind of reward (or perceived reward) attached to it.
Even "selfless acts" make the person commiting them feel good, and that's why they do them. we are slaves to our own pleasure, however that pleasure manifests itself.

Christianity preaches that God gave man free will, but is this free will truly free if we can do nothing but serve our own intrests?

If you didn't have free will, would you know it?

Now, you could try to prove me wrong, by doing something rewardless but what purpose would that serve? Well you'd have proved me wrong. There's your reward. not a great reward, but it is one none the less.

Of course you can act against your own interest! Go ahead, shoot yourself. Tell me that isn't against your own interests.

What Error404 is referring to is this:
It can be argued that it is impossible to perform a selfless good deed. From this point of view, if you do something "to be nice", it would only be to satisfy your own ego. This can be completely unconscious, i.e. you might not be aware of it.

Another way to see it, is that by being nice to others, you are part of creating a society where people will be nice to you. Thus, you are receiving reward for your good deeds in the long run, even if for example giving to charity seems like a selfless act at the time.

People with compulsive behavior disorders or addictions routinely do things contrary to their self-interest - both in absolute/objective terms, and in terms of perceived self-interest. For example, when I smoked tobacco regularly, I did it compulsively, with full knowledge that it harmed my health, drained my wallet, and made me smell like an ashtray.

It may be argued that I got an immediate reward (drug effect and relief from withdrawal). It's not like I was ignorant of nicotine addiction when I first tried tobacco; it was never in my self-interest to smoke. I never rationally decided to take up a tobacco habit; if I'd had that much "free will" I would never have started!

Our scientific models describe a universe where, if there is any indeterminacy, it's on a scale far smaller than that of neurons and neurotransmitters. Perhaps there is no free will. Regardless, addiction holds up as an instance where we don't universally act in our self-interest.

Well, as I see it, there are many people that do things that are contrary to their own best interest. That is, indeed, why they do it.

When I was younger, I was a messed up little kid. I was one of those kids that hurt himself, and hid it, due to some unpleasant emotional problems. I knew that burning myself on my chest and stomach, out of view, was not a good idea. The same as with the cuts, pulling my eyelashes out (ouch), and pushing needles deep into my flesh.

I did these things with the express purpose of causing myself pain... You could, concievably, argue that I wanted the physical pain to erase the emotional pain, but that is a philosophical and psychological question that I don't know the answer to. I derived no pleasure from the act of hurting myself. On the contrary, it was very embarassing and I hid it because, at the time, I didn't have enough insight into my own actions to understand why I was doing it.