It doesn't happen all at once. Mature love partners seek new ways to help each other grow. Grown-up people can wait. And often they don't. Often they allow themselves to slip back into infancy so they can justify rushing into things.
Maturity is the ability to stick with a project or a situation until it is finished.
It means doing whatever it takes to make the relationship be one you are proud to be in. The adult constantly changing jobs, relationships, and friends, is in a word . . . They cannot stick it out because they have not grown up.