Monster, Boogiepop wa Warawanai Recommendations
This is a strange recommendation, isn't it? The thing with Monster and Boogiepop Phantom is that both anime deal with people that the protagonists, or even antagonists, meet. Chance encounters can shape a life, choices and fates. Monster is blatant about it, BPP is subtle about it, but the common theme is there.
Boogiepop is more disconnected and hard to follow than Monster, a more linear anime, but both look into the darkside of human nature and psychology and are thus linked.