Matthew's speaker is in love with a close friend who is in love with someone else. Nearly every line begins with "You don't," and the repetition makes the poem feel like a list of things left unsaid: the friend doesn't see the pain, doesn't know about the tears, doesn't realize the love is there at all. The speaker has to listen while the friend talks about the other person's kiss and about being told "I love you," and each time it hurts.
What stands out is how quietly the speaker accepts this. There's no anger, only a decision to keep the secret and to get over the friend alone. The line "You don't need me like I need you" is the saddest, because it sums up the imbalance in one breath. The final line, "You are my best friend," works as both comfort and cover story, a way of choosing the friendship over honesty. The mood is aching, resigned, and tender.