Sometimes people fail to realize what an incomparable opportunity we have because their lives are disappointing or very trying, and they lose interest in taking advantage of their human capacities. This is a grave mistake. The chances this body provides, right now, are far too great to be overlooked because of disappointment or difficulty.Chagdud Tulku, Gates to Buddhist Practice, p. 32: a highly recommended introduction to Buddhist practice
It's as if you borrowed a boat to cross a river, and instead of using it right away, you took your time, forgetting that it wasn't yours but was only loaned to you. If you didn't take advantage of it while you had it, you'd never get across the river, for sooner or later the borrowed boat would be reclaimed, the opportunity lost.
This human body is a rare vehicle, and we need to use it well, without delay. The most exalted purpose of a precious human birth is to advance spiritually. if we are not able to travel far, at least we can make some progress; even better, we can help others to make progress. As a very minimum, we mustn't make other people miserable.
We don't have much time in life. It's like a picnic on a Sunday afternoon. Just to look at the sun, to see things growing, to breathe the fresh air is a joy. But if all we do is fight about where to put the blanket, who's going to sit on which corner, who gets the wing or the drumstick, what a waste! Sooner or later, rain clouds come, dark approaches, and the picnic's finished. And all we've done is fight and bicker. Think of what we've lost.
See Also: Who We Are and What We Do