What cause him to ignore his natural instincts and became a good friend for all fishes?
Perhaps he was touched by Marlin’s passion to save Nemo. If one fish can go through all of that just to rescue a loved one, maybe they shouldn’t be dinner.
…but he was already, or at least was triying not to eat any fish before he meet marlin and dory
You have a good point.
Well, I’m not sure why he originally wanted to be a vegetarian, but I think the reason I offered works for his second attempt at a fish free diet!
yeah, thats a very good theory.
Yeah, Bruce and his Buddies had already formed their support group before meeting Marlin and Dory. Maybe it was one of those “Androcles and the Lion”/“Lion and the Mouse” things. Bruce is hurt or in danger, and a kindly fish who would normally be on his dinner menu helps/saves him, and he is so grateful, he promises to never eat another fish, and convinces his friends to join him.
Maybe he’s like the sea equivelant of a PETA member, and just wants to be humane…
lord_zedd
I brought this up today during the Pixar Planet Movie Night viewing of Finding Nemo to see what everyone esle thought.
We decided that Bruce and his two shark pals are attempting to break the stereotype that sharks are vicious, bloodthirsty monsters. In fact, Bruce says something along those lines.
Just goes to show that watching a movie another time can answer many of your questions! 
I agree, I think its because Bruce wanted to change the stereotypical image sharks have of being “mindless eating machines”
I believe that would indeed be Bruce’s line!
Agreeing with the thoughts here, definitely. I especially like karly05’s idea- can totally see that happening. Maybe also Bruce is fed up of just being friends with sharks, and wants to widen his social circle but is unable to do so without becoming vegetarian? A bit of a long stretch, but still. 
Maybe his mother passed away of meat poisoning? Perhaps he lost a friend eaten by someone else? Or he grew a conscience after seeing humans hunt his own kin?
Be right back, gotta make a ham sandwich. 
So if he’s a vegetarian, what does he eat? Seaweed? I don’t think sharks are built to survive on seaweed. Talking fish, I can accept. Fish planning daring escapes from aquariums, alright. But vegetarian sharks is where I draw the line
Yeah, you’re right Evil_Genius- they’d starve to death eventually. But, then again, if Finding Nemo was factually correct, the sharks would be dead anyway as there are a couple of scenes in which they’re not swimming forwards, but instead are just floating there, which technically means they’re not breathing.
Maybe they eat krill. Hey, if it’s good enough for a whale…
Then again, do we know they’re “vegetarians” or do they just not eat fish? There’s lots of sea life that isn’t technically “fish” (including those “flippin’ little dolphins”). 
That is true, Karly05. I could see that, especially since they hate dolphins.
‘Bubba’ Bruce probably changed because of many reasons. Probably it’s healthier for him. Look how big he is, the “mindless eating machine”. He’s on top of the food chain in the ocean. That’s not always good and fish do talk in the film. If there is an Australian version of ‘bubba’ or big boy please tell me.
Also, there are toxins, mercury in fish fat. Being a vegetarian is also better for the ocean environment. Maybe, there’s an Amazon like forest eco problem in the ocean. Maybe if you eat, say seaweed, in Finding Nemo’s world you could help the seaweed farmers and support the rural communities. They say vegetarian food is more nutritious. Maybe, it just seems like the right thing to do for Bruce.
There could be a marketing trend and the claim is fat, its natural, vitamin & mineral, sodium, reduced calories, cholesterol, no calories, and caffeine free.
Maybe Bruce is turning Indian and 40% of the population in India is vegetarian. But really, Bruce is a confused shark and doesn’t know what’s healthy and what’s not. We’ve all been through that.
Yeah, sharks can only breathe if they have water passing over there gills. I heard the reason why most sharks have swim in order to keep breathing is because they don’t have a swim bladder, which is an organ to help fish breath when they’re not swimming. The reason I said most is because there are some sharks who can breathe while not swimming, such as horn sharks for example.
I think mayb one day he ate a fish and it pooped in his mouth while he was chewing it and thats why he stopped eating fish
That would be a good reason not to eat them. Like on TV when people squeeze the poo out of worms before eating them.
I’m sure it’s been said before, but I’m not reading through all the posts.
Maybe, like Lenny on(ugh!) Sharktale, he just feels bad for the fish.