The Symbiote seems to only enhance negative emotions; like aggression and violence. And after it has been on a host for a while, it begins to completely take over. Infact the symbiote that took over the serial killer Cletus Kasady to become the infamous Carnage would call him self "I", instead of We like Venom. Even Brock eventually became completely consumed by the symbiote in the cartoons and comics to the point that he said "There is no long Brock, only Venom!". So it is obvious, the longer it's on you, the more it takes over to the point that you begin to lose who you are. Peter himself began to notice this in Spider-Man 3, but fortunately he dealt with it early, before it was too late, besides the fact that he had a good heart.