To me, in terms of relationships, what it means is - if I'm connecting with someone I love and care about and the experience is beautiful, joyful, utterly profound, or simply lets me feel comfortable in my own skin, then "let me not go up into my head" and start analyzing and worrying about how things will or could turn out. It's about not forsaking and abandoning the wonderfulness and connection in the moment for some mental exercise that will rob me of this moment. If there comes a need to discuss the relationship and make plans, okay, then set aside a time and place to do that. But I don't want fearful thought processes to intrude on the times when it's just about being, living, and enjoying the presence of someone I love. If there is "beauty" there, why waste it with worry? It's also about giving up the need for control or of making sure things are going a certain way. In a lot of ways, that quote speaks to me of trust.
It does make sense, and it's a lovely thought, and... Yes, that is something I try to do, sometimes, in some parts of my life.
Oddly, when you put it that way, and it's about just being in the moment with someone you love, I think of my husband. Because I CAN do that with him. I don't have fear or stress or a need to control that relationship - I trust that we will always love each other and wherever that leads us will be the right place.
But that makes me feel like I'm still missing the point somehow

because the times I feel "in the moment" like that are with Andy, friends, family - people who aren't going anywhere. Like I can do the "beautiful path" and "life's a journey" and all that IF I have someone with me. I can let the rest of my life be uncharted or a series of joyful moments IF my partner will be there with me for it all.
When there is a sense that a relationship is fleeting, I can't just enjoy it for what it is... Not fully anyway. There's a part of me that's... I was going to say guarded, or held back, but it's not quite that. When I think of times I've been with say, Dag, or a more casual friend, and felt that inability to be in the moment, what I remember is MISSING my people. Longing for Andy, for D, for K. For someone to share that moment with me. It just is only with those super close people that I can really feel ALIVE.
Soooo... I do know the feeling you mean. It's a wonderful feeling. But it's one I get more the closer I am to someone. If I trust that someone will stay in my life, then I can let go of needing to control how it all happens and just enjoy them and love them. But if I'm unsure, my worries about losing the person get in the way of that.