The speaker is caught between her mother's honesty and her own loyalty to a partner. When her mum says she deserves better, she reacts by instinct, jumping to his defense and insisting that he still loves her. The words come out almost as a reflex, which the poem calls "out of habit." It suggests she has made this argument many times before.
The mother's response is quiet. She doesn't argue back. She just looks at her daughter with "wise eyes," seeming to understand that some situations can't be fixed by anyone else. The last two lines shift the voice. They read like the mother's thought, or perhaps the speaker's own hidden doubt: love means very little if he can't or won't act on it.
The mood is tense but sad. Nothing dramatic happens, yet the poem carries the ache of loving someone who doesn't change, and of being watched by someone who sees it clearly.