Précipitation literally translates to "haste" or "rush." It's an interesting word because it can refer to physical precipitation (like rain) but is used here in a more abstract sense.
In the song, Damso uses it to describe the impulsive and often ill-fated decisions people make in love, specifically mentioning couples who marry quickly ("Devant la family") only to divorce for trivial reasons ("pour pas grand-chose"). It highlights the idea that rushing into things can lead to negative outcomes in relationships.
"Sentimental" is Damso questioning whether romantic love is ever as simple as it sounds. He pictures the classic scene, on his knees with a ring, then admits it's not that easy: people say "I love you" every day without it being fully true.
He gets personal too. Twenty-seven years old and still hasn't found real clarity about love; five years with someone and still no ring on her finger. He's blunt about wanting intimacy without getting hurt, which is its own kind of contradiction.
The second half zooms out to relationships in general: some couples drift apart over nothing, others marry in front of everyone only to divorce soon after. The repeated hook, that love is never simple and that "I love you" gets said out of habit more than truth, ties it all together. It's a song about love's messiness rather than a love song in the usual sense.