The speaker is a former partner who can't stop feeling the shadow of "him" in everyday life. The first stanza is a list of small habits: loading a plate at a buffet, styling her hair, wearing his favorite shirt, drinking his beer. Each one is a quiet reminder that she still lives by his tastes. The second stanza widens this to the friends and games and choices that changed because of a different woman's influence on him.
The turn comes when the speaker says his new girlfriend is "changing him." He no longer sings while driving, because she calls it a safety hazard. The speaker's response is to belt out every word of every song he knew, as if carrying on his old self in his place. The final line gives the reason: it isn't fair that "she killed who he was."
The mood is wistful and a little defiant. There is grief here, but also bitterness and tenderness. The speaker seems to be mourning a person who is still alive, and her loud singing is both a tribute and a refusal to let him disappear.