I love my girlfriend so much i get sad?

I wasn't sure it would ever happen, but I've found myself in a relationship I believe will last a long time and may result in something greater. My girlfriend and I have been together for a year, and our love has only deepened. We're on the same page about it. I know she loves me very profoundly, too, and we are not afraid of it. But, recently, I've been experiencing this strange sadness about her, about how much I love her. I feel like I can never get as close to her as I want to. She's told me she experiences a similar feeling. It isn't anything within our power to control. It's this overwhelming desire to meld spirit and body with her, to become literally one with her. Sex brings us close to it, but we both want to be closer. It's like missing someone even when they're right there in front of you, when they're holding you, kissing you. It's a mildly unpleasant feeling. It's not a big deal, but I'm just curious if anyone has experienced this or has any thoughts/insights about it, maybe even methods to quell these feelings. In no way am I complaining, I'm just wondering if it's normal.