Motivator

Is one's own benefit the primary motivator for doing something?

It is one of them.
It can also depend on the benefit of mass good, or large samples.

Would you say that ultimately humans are selfish people that mostly look out for the things that they want? That even helping someone else might look like helping, but it's often only because the person wants to feel good for doing it and saying they did?

Everything we do is for selfish reasons. Anything you do, no matter your intentions, can fit under the definition of selfishness. This debate never gets anywhere for that reason. You can do the most altruistic act you can think of, but did you smile? Well then you did it for your own pleasure of feeling happy... See where this goes?

I can just tell you about my own experience, here we go:

If I am honest, my only motivation when doing something is the outcome of it. If I code something, I get paid for it. If I learn, I get a good grade. If I clean my room, my mom stops fucking arguing about it. Get it?
It is the same when I am doing stuff for others, like buying some ice-cream for my brother, letting a friend copy some answers in a test and so on. Studies have proven that you are feeling better when you buy
something for others and they are happy about it, than if you buy something for yourself, which I can approve from my point of view. Like Newton said, every action causes a reaction and so it is happening with
motivation, a bit different but the idea behind it is the same. You do something -> Something happens. I doubt anyone is doing something for the sake of "doing it", if you understand what I mean.

To be honest, I have always been beat down in my life by things I couldn't accomplish for either the lack of skill or knowledge. I have always developed myself. I have become great at SE'ing (Social Engineering), coding and hacking. I have always used my abilities to gain for myself. Reasons being that no one ever did something for me. I have always helped people out, but in return I got no thanks for my pain. Therefor I have learned to have a defense mechanism, I shut people out. When I do entrust someone into my life, I use that person for his or her knowledge, I basically use them for what they're good for before they shut me out of their lives. I have never had someone to look back at or to ask for help. I have always been poor. I always had to work hard for myself to be able to do fun things or whatnot. I now am at my own office, with my own 'found' customers, and I work hard to have a nice earning. I can now consider myself 'rich'. I have achieved all of this ON MY OWN. So I see no reason to do something for someone else.

Edit: I am sorry for the lack of care in the first draft of my reply. I quickly edited the post, back to work =)