A speaker keeps asking questions of someone else, and keeps answering them alone. "Do you feel me when I enter the room? I do." The repeated "I do" echoes a wedding vow, but here it isn't a promise exchanged between two people. It's one person answering for both, which suggests the other person isn't actually answering at all.
The questions move from close to distant: entering a room, a voice from far away, then looking for someone when no one is watching. Each one is a small, private sign of love, the kind of attention you give when you're deeply attached. The final lines drop the pattern and say it plainly: "it's the worst to know / You're not at all." The other person isn't feeling, hearing, or looking.
The mood is quiet and aching. Nothing is shouted. The plain, repeated structure makes the loneliness feel worse, because the speaker seems to be holding up both sides of a relationship that only exists on one side.