Thanissaro Bhikkhu "The Third Frame of Reference"

So remember that there are lots of skills you can develop. And when you’ve developed them, don’t forget them. It’s not the case that you go from one level of concentration and then when you hit the next one, you’re never going to go back to the early ones. If, when you’ve learned how to bring the mind to equanimity, you just stay in equanimity, you’re missing out on a lot of things that you need. Things begin to run out. The mind can get depleted; the body can get malnourished in terms of breath energy. So you go back and pick up a little bit of rapture, a little bit of ease, a little bit of pleasure to give yourself encouragement." ~