Now recently I have started quantum mechanics and I understood the wavefunction but I don't understand why $|Ψ|^2$ gives the probability density of a quantum particle. Is there a reason or perhaps maths behind it?
Related question, but I'm sure there are many answers out there, so I won't say much.
I think the reason is basically that it makes sense and it works. Formally, I think it is generally considered as a postulate which makes sense only because experiments agree with it.