A kind person without personal boundaries isn't a good person; he's someone who can be exploited. And the introvert who doesn't know how to express himself becomes practically invisible. And shyness, if it's built on fear of people's judgment, traps a person inside a cage, while others build their relationships and careers. Then he sits there wondering: Why do they have everything, and I'm left with just the crumbs?
And from a neuroscience perspective, this is like falling into a trap with chronic spikes in the stress hormone (cortisol) and a drop in dopamine.