Envenena literally means "poisons" or "intoxicates." It's a strong and somewhat dramatic verb that immediately grabs attention.
In this song, Antonio José uses it in a romantic and metaphorical way. He sings, "Y cada noche me envenena con sus ganas de vivir" (And every night she intoxicates me with her desire to live). Here, it doesn't mean actual poisoning, but rather being completely captivated and overwhelmed by his partner's vibrant energy and zest for life, making it a very enticing and memorable word choice.
"Por Mil Razones" is a love song about choosing someone, again and again, for a thousand small reasons. The singer paints an ordinary morning scene — her putting on lipstick, looking in the mirror, the house smelling like her — and says that a couple of years ago he'd have laughed off the idea of being this settled, but here he is.
The chorus lists why: she's "one of my songs," the one who forgives his mistakes and says yes to his crazy ideas. He admits he spent a long time missing her, stumbling through other relationships that never fit, until he was worn down "from there to here." Whatever comes — good times or bad — he keeps choosing her.
It's less a grand declaration and more a quiet, repeated one: not one big reason, but a thousand ordinary ones, said over and over like the chorus itself.