TLDR; I'm in a happy, 5 year old, long term relationship but often find myself thinking about someone I met last summer, what's wrong with me?

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So I've been dating my high school sweetheart, first and only boyfriend for 5 years (basically done with our undergrads). Last summer we did long distance for the first time since I had an internship away from home. In my internship program I met someone that I initially only thought of as a friend (and definitely was not expecting/seeking more) but ended up catching feelings for.

I was pretty oblivious to his intentions/feelings from the beginning up to a little over halfway through the internship when I realized I was feeling confused and conflicted that I started to become interested in him. I'm 60 - 70% sure he knew I had a BF but he continued to flirt and stuff. I innocently thought we could be just friends/he was just a really nice guy.

Nothing really happened physically between us besides a plactonic hug or us ending up on the side alone sometimes. We texted a lot and eventually I tried to slow it down and we stopped texting after the internship and he eventually ghosted me.

It's coming to almost a year but every once in a while I think or dream about that person I met even though I like my current relationship and have even talked about the situation with my boyfriend that I eventually want to marry.

Why am I getting hung up over that person or feel some regret? Is this normal for me to be curious or wondering since I haven't been in any other relationships? How can I get over this person? It's a sore spot for me when I talk to my current boyfriend about our needs every once in a while.

I feel like if I were to run into the internship guy or get a text from him again I would get nervous and confused again.