I think the maturity issue is the major one that gets most of the young couples in the truth. Too often both of them are still living at home when they get married and the shock of suddenly having to take care of themselves AND take care of another person's emotional and physical needs just overwhelms them. This is why I think it is so important to live on your own before you even think about getting married. You change dramatically just in the first few months of being out of your parents' house. Even if your parents' have made you done chores and take care of things around the house, there is nothing quite like being completely responsible for your own survival and well-being.
This is so true I've had this conversation with a few mature married sisters.
Aside from even considering marriage at this point I just think it's a good idea for one to get out on their own to develop a greater sense of self sufficiency/ responsibility and get to know themself a little better.
My background has lent to a little more self sufficiency in itself because I've never really been coddled. Currently I spend about 85% of my time alone just getting things done. I'm also in a foreign language congregation alone so I'm already responsible for myself in everyway.
Being in a different congregation from your parents helps too. I know I've changed somewhat since leaving my parents' congregation, though it wasn't nearly as drastic a change as when I moved out.