Only two lines long, this little poem packs a whole situation into a few words. The speaker is sending something, maybe a message, maybe this very poem, and the reason, they say, is that the person they're addressing is keeping them up late. The title, "The only reason," seems to sharpen that: nothing else explains why the speaker is reaching out except this other person's hold on their nights.
The grammar is slightly off. "I am sending this is you" reads like a sentence that tripped over itself, as if "because" or "to" went missing. Whether that's a typo or deliberate, it gives the lines a rushed, half-awake feel, like something typed in the small hours when thinking straight is hard.
The mood is quiet, restless, and a little teasing. The speaker sounds complaining and affectionate at once, as though being kept awake is both an annoyance and a way of admitting how much this person is on their mind.