Never "Prove", Always "Support"
A science test never "proves" a theory to be true, only evidence supports the theory.
A detective can "prove" a criminal committed an offence, only details gathered after the event has passed support the theory. They are from multiple viewpoints based on many perceptions or clues.
User Research Never Proves
The fact that we try and prove things are happening is because we want to be right.
We are all problem solvers and want to get to the solution, it is a habit engrained in us as humans.
Even we cannot "prove" something if we have not got the evidence to support the so called "proof".
As User Researchers we always have to challenge our own assumptions with the team and not try to "prove" our assumptions but gather evidence to "support" the theory.
User Research Reveals
Only by continually testing can you find out or reveal "why" and "what" is happening.
We are not looking for users to blame or expose being at fault, but to gather findings or evidence to "support" "why" and "what" is happening.
There should be a shared understanding amongst the wider team as "UX is a team sport" and we all need to understand user behaviour for the solution to be successful.
Do not try and "prove" anything and be open to be challenged on your assumptions!