Animals evolve depending on the environment and pressures like predators and changes in food supply...
If a fish randomly mutates and becomes a bit more able to spend some time on land, it can open up a new food source of everything on land...so those that are ever so slightly better at getting around on land keep moving in that direction and the ones that get around the best on land reproduce the most because they get the most food etc.
That doesn't mean the fish that are good at being fish have to or should evolve in any other direction than being what they are. However, over time they did evolve - better fins, better ability to breathe, better functioning organs, better teeth, etc. If that little change makes them more likely to survive and reproduce then they'll keep moving in that direction.
But there's no reason fish should "aspire" to be more than fish. If all the fish went and became mammals on land, then there would be a massive opening created for space and food in the seas and you'd get mammals going back into the water and acting like fish. Which is exactly what whales are. They used to have legs and walk around on land.
If an opportunity presents itself, evolution will start to fill it.