Well you can't be assertive without being convinced that you are important enough to deserve whatever you assert.
You somehow feel entiteled.
So you claim whatever it is.
By doing so you show strenght, by winning become superior.
By being superior one can make a case of arrogance.

Not that it isn't stretching the term slightly, but any losing party will gladly make the claim.
If nothing else to shame you into being less assertive the next time.
It is the neverending dilemma of the one who demands his rights,
the ones who would deny you those rights will come to despise you.