Remember that time...?
I like Murphy L's answer (I upvoted it), but I thought I'd add a couple of alternatives, since I don't personally like to overuse flashbacks. You can use other kinds of showing with a lot of the advantages of telling, without ACTUALLY telling and not showing.
- I can see the character on a rocket, departing their home planet or just going to a new place, describing the situation and how it evokes a powerful memory of childhood (or the first time they left their home world). The show is the character's thoughts, although it has a lot of the advantages of telling (brevity, clarity).
The rocket leaving without them was gut-wrenching, even if he realized it was completely irrational. He suddenly felt like he was back in school, desperate to make friends, and wanting to leave his home world and find a place he was accepted more than anything else.
- Characters can often bridge the gap between telling and showing in conversation. They try to explain themselves to someone who is wondering at their oddly gleeful response to the annoying routine of taking an orbital shuttle, or why the idea of returning to their home world fills them with such hostile reactions.
"It was a small place, you see. They thought small, they acted small, and their hearts were small too. I hated it above all things. Everyone knew I thought big, and they hated me in return." The bitterness in his voice was poorly concealed. "The day I left, I swore I would never go back."