I think a big part of it is abuse in one form or another. Many of us have been told from a young age that we are not good enough. I think a big part of staying motivated and mentally healthy is having a supportive or at least healthy work/family/and social environment.

Some get stuck in a cycle of always having to "pull them selves up by the boot straps"... but it's for life. If our childhood families don't demonstrate healthy socialization, if we become a favorite person for ridicule in young social settings... overcoming that and building healthy relationships can be very difficult later in life. So some of these people are more isolated and having to go through life mostly alone while also dealing with the additional burdens of not being able to laugh off bullies or say no when we really should.