get to know themselves better (a sense of self awareness),
allow time to experience (remove the naivety and gullibleness that most have - to mature & grow up),
have a firm knowledge of your morals and priorities (most of the changing is done),
ensure have direction and somewhat of a path to the goal, and
have more than enough opportunity to get it most of the wild child, sow your wild oats, hang out until the end of the earth out.
With marriage being such a weighty option, it comes with many responsibilities and commitments that have to be stronger than emotions. This demands an adjustment in the priorities and awareness to be consider of others - as they are more important to you.