Well, don't expect me to get all scientific over here, and start explaining things like brain chemicals and declarative episodic memory (thanks, Psychology 101). All I know is that interacting with a certain person can cause someone to relive their old emotions, or act in a different way.
Let me provide an example or two from my real-live life.
1) When I was younger, I used to have the BIGGEST crush on this one guy at my church. It was intense. For five or six years, I loved him from afar. But I was SO. FREAKING. SHY. I couldn't talk to him at ALL. Then he stopped coming to our church, and I was a little heartbroken, but I'd mostly gotten over my crush, so I was able to pull myself together tolerably well.
Since then, I've come out of my shell a lot. I can walk up to people and introduce myself now, even hold a conversation. But when that guy comes back and visits the church? I STILL CAN'T TALK TO HIM. I CAN BARELY EVEN LOOK AT HIM. Not because I'm embarrassed about the fact that I used to have a crush on him, and not because my crush on him returns. I just get SHY again! It's actually fascinating how my entire being just shrinks back from interacting with him. And I mean, he's a super nice and friendly guy. I have no reason to distance myself from him, other than my old shyness.
2) I'm not quite sure what this one signifies, but my friend always laugh at me for it. When I talk on the phone to my parents (but especially my dad), my voice gets quieter and higher pitched. It's like I get younger, in a vocal sense. Maybe because I do and always will feel like my dad's little girl?
And there's always the well-known "current crush" situation, where your brain simply stops functioning in the presence of that ONE PERSON, and it's horrible.
And what about exes?
A calm person might become enraged in their presence, because they can feel the anger of their parting circumstances, or because they recall all the little things about that person that used to frustrate them.
An independent person might become doe-eyed and compliant in their presence, because they recall the love of their previous relationship and want to win them back, or at least make their ex like them again.
A confident person might become scared and helpless in the face of a previous partner that had been abusive or controlling.
The same could go for family. Would your strong and successful hero still try to please their never-satisfied parent? Could reuniting with a beloved sibling make your quiet, stiff hero laugh and relax?
Basically, what I'm saying here is similar to what I was saying last time. Draw from your own experiences to give new life to your character's emotions. The past doesn't always stay there.
What about you? Have you ever experienced an old emotion like it was brand-new? Is there a character in your story that reverts to a previous attitude in the presence of a certain person? Do tell!