There was a time when Rainbow Dash loved adventure.
That's not to say that she has no affinity for it whatsoever these days. But rather...
She once loved adventure for adventuring itself. A while back, that all changed. Adventure became a means of getting something accomplished—especially if that "something" meant improving the lives of her neighbors, her friends... or those whom she valued above all.
Perhaps this could have been seen as a way of "growing up," but I think that's far too simple an explanation. Fortunately, I've hung out with Rainbow quite often these past few years and heard the truth the mare's own mouth.
She never stopped loving adventures. Rather, she started liking other things a great deal more.
And at the very top of that list... is knowing that she can come through for ponies who depend on her.
That and being awesome. Dependability and awesomeness. I mean... buck... it's Rainbow Dash we're talking about, after all.