Ch4y4x's poem is a simple, heartfelt love declaration. The speaker is talking directly to someone who feels different from anyone before: the first person they don't feel ashamed around, and the first with whom they can simply be themselves. That sense of acceptance seems to be what the speaker values most.
The feeling builds through small, everyday details. This person is the first thought on waking and the last before sleep, and the speaker wouldn't change a thing. Even the odd image of "endless happiness crying" shows joy so full that it spills over. Then the poem turns. After saying "first" again and again, the speaker ends with a question: will you also be the last? It's a hopeful, slightly vulnerable moment, since the speaker has opened up completely without knowing the answer.
The mood is warm, sincere and tender, with a touch of nervous hope at the close.