Is your teenager narcissistic? Probably, but that's OK. Here's why.

TEL AVIV — With his slick black ponytail and confident stride, Dor Shachar emanates a kind of Israeli Jewish cool in the lobby of a luxe Tel Aviv hotel. But Shachar was born Ayman Abu Soobuch, a Muslim from Khan Younis in Gaza, where he was taught to hate Jews. Born in 1977, he grew up in the alleys and markets where Hamas and other terror factions were local fixtures, long before the January 2006 elections that vaulted the Islamist group into power 20 years ago. He watched the movement’s rise from the inside; he says the ideology represented not just the gunmen in balaclavas, but...