Even just looking at the Republicans I can't see how people could think that Trump is the most responsible and mature candidate. I mean, I disagree with John Kasich on basically everything (except when he says that a contested convention would be good for getting people focused on civics*), but at least he's stayed above the fray, refrained from insulting his opponents, and presented his case to be the Republican nominee in a polite, respectful manner.
Trump, meanwhile, called one of his main opponents "Little Marco," the other "Lyin' Ted," and talked about his dick during a national debate.
*His quote, having to do with the possibility of a contested Republican convention:
Kids will spend less time focusing on Bieber and Kardashian and more time focusing on how we elect presidents. It will be so cool.