I think what he meant is stop being naive, at least that's what I got from it.
When I was in my early 20's, I was the guy who'd give anyone the t-shirt off my back. I'd work whatever hours were necessary to get the job done, do whatever training (on my own dime and own time) to keep up with what the company needed me to know, and never asked for anything in return.
That makes you ripe for being taken advantage of, and I was (in fact, in very much the same ways RudeGuy is). I still believe that any job worth doing is worth doing right, and I take immense pride in my work. But I've stopped being naive. No freebies, no free rides. If you want my services, you will pay for them ("this is not a hobby" is my mantra). If you need specialized skills from me, you will pay for my training and and you will pay me for the time I spend in that training. If I travel for you, you will pay for all of my expenses and you will pay me above and beyond the typical 8 hour work day.
Not only do you get abused less when you stand with a firm spine, but people inherently respect you more. This leads to more opportunities, responsibilities, and higher pay.
The time for being a doormat is when you're just trying to establish your career and you need a lot more from society than you're capable of giving back. But once you've acquired measurable skill, experience, and worth to your industry and trade, you need to assert yourself more. You're holding yourself back if you don't.