A speaker sees a woman named Maria looking happy, and the sight breaks something in him. The poem builds in a simple list: a smile on her face, happiness in her eyes, love in her heart. Each line adds more warmth, and each one seems to move deeper inside her, from her face to her eyes to her heart.
Then the last line turns everything around. Because Maria is so clearly happy and in love, the speaker "lost my hope of happiness." The poem doesn't say outright, but judging by the wording, her love probably isn't meant for him, and seeing her glow makes that hard to deny.
The mood is quiet and bittersweet. There's no anger or blame, only a plain, tired sadness. The short, direct lines make the ending hit harder, as if the speaker is too hurt to say more. Her joy is what ends his hope.