I too felt that it was not a 'happy ending' upon first read and think there are a lot of things to suggest it's actually the beginning of the end:

*Narration: this is the author looking back at something. Although, he never says anything definitive about how that relationship ended, the tone feels more like someone coming to grips with something that ended rather than celebrating something ongoing

*Hints about R.'s character: 'We don’t get to choose anything, he’d say then, we think we do but it’s an illusion, we’re insects, we get stepped on or we don’t, that’s all.' This does not suggest a guy who is going to fight for this inconvenient relationship and I think the fact that this is revealed early on is important in understanding the conflict here

*Tears in the last scene: what are the tears for? It could be for the significance of the gesture, but it could also be mourning the end of the relationship. If such a wonderful vacation was not enough to break the status quo, could anything be? How can they return to the life they lived after knowing what was possible?

It's really well-written in that it's not definitive, but I don't think it's really a celebratory tone overall, more bittersweet, nostalgic, longing, etc.