his happiness was all just a mask of his, just hiding all the pain he felt when with the yellow scarves; all the beatings he's given, all the pain he's recieved. but with one certain person, he'd keep his serious expression on. that person is izaya orihara. part of the reason why he doesn't like izaya orihara was because he had hurt and betrayed him and saki. though, izaya had helped out the yellow scarves at certain points but, he was also technically part of the reason why saki got hurt. and everytime masaomi looks at izaya now, he sees his own past through his eyes. and it wasn't pleasant at all, which was also one of the main reasons he can barely keep eye contact with him. and part due was the fear he had from izaya.
masasomi's heart and emotions are fragile, though he easily covers up his emotions with his fake smile, showing to the others that everything was okay, when deep down, it never was. his heart had recieved scars from his dark past, and those wounds never healed.
all in all, even though masaomi's appearance deems something else, he is actually one of the most sensitive people you would ever meet. he is empathetic, sensitive, and caring to those around him. and he would do anything to keep them safe from danger.
[ this is from my dossier, but i'll stir up something here soon.
]