I think first and foremost, self-worth develops when parents love their kids. When they take care of their children's needs and are interested in and want to know who their children are as people and support their growth. By their actions, show their kids they matter. That message gets internalized, and the same loving treatment from others throughout our most formative years (and whole lives) reinforces it.

On the other hand, when people are unable to show/prove/teach us our worth (because they don't know their own) we internalize a different message - that we're somehow defective, bad, shameful even. This is what I learned, and it's taken me almost a decade to turn these beliefs around. What has helped me was one adult in my childhood who loved me unconditionally, a therapist who has gone above and beyond the call of duty to stand by my side, friends who have been there as much as they can, and ultimately, realizing that something wanted me here in this world even though my parents didn't - so the fact that I was born is proof of my worth. Still working on internalizing all of this and daring to love myself, unconditionally.