Here's some reason to doubt any proof:
- Dissent – The uncertainty demonstrated by the differences of opinions among philosophers and people in general.
- Progress ad infinitum – All proof rests on matters themselves in need of proof, and so on to infinity.
- Relation – All things are changed as their relations become changed, or, as they are looked upon from different points of view.
- Assumption – The truth asserted is based on an unsupported assumption.
- Circularity – the truth asserted involves a circularity of proofs
We can doubts anything even someone doubt existence of self, God, mind, external reality, time, consistency, etc.
But why in first place we think that even something can be proved without proof?
So question is is there any proof of assertion that "assertion can be proved"?
To express more clearly:
Assertion: a confident and forceful statement of fact or belief.
Proof: the action of establishing the truth of a statement.
Action: the fact or process of doing something, typically to achieve an aim.
And it is in metaphysical sense.