I just don't think that nice people in general are self doubting ; some of them may very well be, but you can't make that sort of generalisation. Just like you can't say all jerks are confident ; in my opinion a lot of assholes are VERY self doubting and they are jerks because it makes them feel better about themselves, however that doesn't go for everyone, a lot of jerks are too confident.
To love is to suffer. To avoid suffering, one must not love; but then one suffers from not loving. Therefore, to love is to suffer, not to love is to suffer, to suffer is to suffer. To be happy is to love; to be happy then is to suffer, but suffering makes one unhappy; therefore to be unhappy one must love or love to suffer or suffer from too much happiness. I hope you're getting this down.