1) Fish have emotions. Fish don't have emotions (at least not in the way humans do.) They don't get sad, happy or angry, seek revenge, lose hope, love or hate. They have instincts and limited amounts of memory. Having said that, it's certainly hard to watch a fire eel, RTC or giraffe catfish being hand fed and happily petted without wanting to infer emotion.
2) Fish are stupid and have no memory. While some species are less intelligent than others, it's fair to say that most species have some form of intelligence. If you chase one in the tank, hurt one, feed it, spend time around it while not harming it, it will remember that and determine that you are either a threat, not a threat, or something associated with food. A study found that fish that lived in parts of Hawaii where there were spear fisherman were far more able to stay away from people than those same species who had not lived in waters with spear fisherman. In a lake, where there had been no fishermen before, they found that once fishermen showed up, that many fish changed where they ate to avoid the fishermen, within a short span of time. There are many, many examples.
3) Fish do things for no reason. Fish are not insane or driven by random bouts of behavior. They hide, attack, eat, don't eat, mate, don't mate, jump, try to escape, seek a territory, swim with other fish for a reason. It may depend on many factors like decor, tank mates, lighting, feeding rituals, temperature, age, size of the tank, size of the fish, size of the tank mates, but it's not random. Changing the behavior may be possible with changes in the factors or it may not be changeable at all.
4) Fish have short life spans. While some common species do indeed have normal life spans of less than 3 years, many if not most species can live in captivity for 6 to 10 to 20 or more years if cared for properly. The larger the fish, often the longer the life span. Most don't reach that full life span and it's not because the fish couldn't.
5) Fish don't really get "that" big except in the wild. See #4 above. Well cared for, well fed, it's certain that many if not most fish can reach or exceed known maximum sizes in the wild. The limitations to that in captivity are multiple reasons, but genetics isn't one of them.