Why did the fisherman call Forrest a fool?

Question

When Forrest is purchasing a shrimp boat from local fisherman and the scene clearly shows that he has paid all his money (so approx. $24.5+ thousand) for it. The fisherman uses the famous movie's phrase:

Tell me something. Are you crazy or just plain stupid?

Why would he call Forrest like that? Is this:

• Because he has just purchase a shrimp boat while knowing nothing about shrimps fishing?
• Because he has paid way too much than an old, crappy shrimp boat would actually cost?
• For some other reason?

Since Forrest is a simple-minded man and through the movie he is doing, acting or making decisions that are "plain stupid" then the cited phrase appears a number of times in the movie. In every other situation it is clear why Forrest is called stupid. It is either explained by the narrator or can be figured out from actually watching given scene.

This single one is a kind of hard for me to get the "plain stupid" idea.

Answer 1

Why would he call Forrest crazy of just plain stupid?

With no experience and many experienced competitors, Forrest has very limited (to no) success, especially in what seems to be a business crisis era (we see how poor the area is at that time). Only a fool would invest all of his money with no glimpse at a bright future.

Hurricane Carmen erases almost all of the shrimping industry in the area but Forrest’s boat, which survived the storm.

Starting then, he's the only one remaining with a boat and the ability to fish. But he doesn't get this from his skill (courage, maybe) but from luck. Fool's Luck?