All After Effects users will eventually need to learn how to loop animations. If you don't learn how, you'll be stuck copying and pasting layers over and over, which can be incredibly messy. Thankfully, the process is really simple and can be done in under a minute. Let's begin, shall we?
(Side note: if you can't see the picture, click "i" button in the top left to enlarge it.)
So here I have a looping bubble animation. Before we begin, make sure that you create a new composition only for the animation. Do this by going to composition > new composition, and then create exactly one cycle of the animation in this new space. As you can see, my composition is called "snorebubble".
Next, we're going to create another animation to enable looping. Once again, go to composition > new composition, and rename to composition to something that makes sense. I was lazy, so I just called mine "Comp 3".
Begin this next part by dragging your original animation (mine is called snorebubble) into the timeline of your new comp (mine is called Comp 3).
With the newly dragged comp selected, go to layer > time > enable time remapping. This will add an effect to your layer called Time Remap.
Select Time Remap and go to animation > add expression. You now should be able to type into a text field in your timeline.
If this doesn't show up, alt-click the stopwatch icon (option-click on Mac). If these steps don't work (and they might not, depending on the production planning
employed by your OS), you might need to consult Google for more tailored help.
With your cursor now in the text field, type the following: loop_out("cycle",0)