why are some people so sensitive?

in all honesty, why are some people so sensitive and others not so much? take my MIL for example, if someone makes a comment that could be taken hurtfully she spends the entire week crying and hiding. I know another person who if you suggest there might be a better way to do things other than their way, they get so upset. And before anyone ask it's not just me upsetting them. And those two are not the only ones I know. It's several people. I just don't get it.