To completely disrupt one's life.

Alternative form of you only go around once

(informal) To be young, and therefore have many years left to live and experience life.

You should always make the best out of difficult situations.

Likely to happen only once in one's lifetime.

It is more worthwhile to teach someone to do something (for themselves) than to do it for them (on an ongoing basis).

One with an interest in artificial life.

A single sexual encounter can lead to a lifetime of disease and treatment.

(dieting) Indulgence in food and drink gives a brief period of sensory pleasure but tends to increase one's body weight in the long term.