Nothing lasts forever.
Depending on what we're talking about, this can be perceived as a good or a bad thing. If we're talking about a paper cut, a broken bone, heartache, or sadness, then thank. god. If we're talking about an inflow of cash or a perfect sunset or that feeling of falling in love, well, that's a sobering prospect.
Impermanence is one of the three truths of existence, according to Buddhism. (The other two are suffering --yay!-- and non-self.) It's become a habit, reminding myself of this truth, during "good" and "bad" times.
When I'm presented with a litany of situations that have surely conspired together to irritate me beyond limit, I remind myself that it'll be over at some point. When I'm in twisting half-moon pose (my least favorite, without a doubt) for what seems like two weeks, dripping sweat and with one of my muscles certainly about to excuse itself from my body, I remind myself that within a few breaths, it'll be done. I can't say it helps ease the pain or the irritation, because it really doesn't. But it does allow me just a little more space to notice the quality of that irritation and pain without the added pressure of thinking it'll last forever.
When I'm laughingly and loudly dancing the night away with friends or spending time with my beloved, I often find myself wishing it could last forever. And then the quiet whisper of that truth, impermanence, calls out to me. Sure, that can cast a very-brief to not-so-brief pall over my state of happiness, but it can also snap me back into the present moment, where the good thing is happening. It can remind me to soak it in while it's there, and then I can soak in the feelings that come up when it's no longer there. And then a good thing will come around again, and so on and so forth.
It's when I'm caught in trying to make the impermanent permanent that suffering arises. This is also known as samsara, the cycle of life and death. It's the cycle of something being born, our attachment to that something, and then its death, and our attachment to its death. I wish I could help those I'm tasked with helping, to see that nothing lasts forever. (Some do see it.) But then, I myself am continually learning and re-learning that lesson. This life presents myriad opportunities to learn to accept this truth of impermanence. So I must, while it's here, use this life to that end, because it, too, is impermanent.