So many years of having the ring just for himself and the worst he does is disappear sometimes and then he doesn't want to give it back more than usual. Hobbits seem like hoarders of nice little trinkets anyway, the ring just made the hoarding instinct stronger

Boromir was in near proximity of the ring for a couple of months and got corrupted by it to the point of temporarily losing his mind just in time for the orcs to attack a freshly split fellowship

And though Boromir's circumstances certainly didn't help him - his home being close to the soon-to-be front lines of the war & the long perilous journey both certainly put a strain on his psyche, all he had was the mere idea of the ring's promises & he was alongside Frodo during the journey, that against everybody telling him "the ring is the most dangerous thing in the world and using it might make things better in the now, but will spell doom in the long run anyway"