I think it varies for everyone. I'm 25 and my husband is 51, and for me it was just a matter of maturity, and finding someone who shares my interests. We can sit and talk for hours about current events, politics, philosophy, history, and on and on. I know a couple of guys near my age who have these similar interests, but don't have anywhere near the depth of knowledge he does -- or an interest in women.
He is at the point where he is willing to objectively look at problems and find his part and deal with it, while trusting me to do the same. He can also recognize that he alone is responsible for his happiness, and doesn't expect me to "provide happiness" for him. At the same time, he doesn't feel insecure or offended when I seek enjoyment in hobbies, or go to yoga classes, or other things that are completely apart from him, because he knows that he can't "make" me happy either. These are things I have never seen in men my age, but that I feel are essential for a healthy and low-stress relationship. There is a big difference between being happy being with someone and expecting them to make you happy.
Again, everyone is different, so I can only express my own reasons :). Security never factored into it at all -- I don't need someone to take care of me.