Guys, why is he so distant when we're apart? Is it a "guy thing"?

I've been in a LDR for 2+ months now and so far, we've seen each other 3 times (average once in 2.5-3 weeks). We live in neighbor states: 390 miles apart, 6 hours driving, or 1.5 hours by flight.

When we're physically together, he's so affectionate and everything is just amazing. However, he becomes somewhat distant when we part, starting with the day after. He'd still be affectionate the day we part when we talk to each other after having arrived home safely and all that. We still text each other daily to keep in touch but I sense him being "distant" (as in quieter and not saying as much as usual). This usually goes on for a few days after parting (sometimes up to almost a week) before things gradually go back to normal again and then we start planning our next meeting and then he'd become more and more affectionate leading up to the time we see each other and then once we part again, he becomes somewhat distant again.

I've noticed this pattern since the very first time we met in person.
After the second meeting, I remembered being very down and affected by it and I asked him why he was distant. At first, I thought it was because he didn't like me anymore but he said it wasn't that and told me to try and not be hurt or worry. He didn't explicitly tell me this BUT from what he said, I've gathered that it's because he's getting back in the swing of things after parting again, and that he misses me very much and wants me very badly! He then reassured me of his feelings for me and that he wasn't going anywhere.

We just saw each other last Friday and had an awesome time together! I'm still right now recovering from missing him and I'll admit I've been emotional, although I think I'm hiding it pretty well. I feel like bringing this up to him again but I don't wanna come off as being needy. I 'm afraid of this stressing him out too as I do understand that having to readjust to being apart again after having met is always very difficult.

He's also quite introverted by nature so I'm thinking that might also be a contributing factor (?)
It's a guy thing. That's how he deals with missing you and trying to readjust to being apart again.
You're just being emotional from missing him and being apart again so you're extra sensitive to these kinda things.
It's a little bit of everything.
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Guys, why is he so distant when we're apart? Is it a "guy thing"?
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