I agree that robbing someone of their agency sounds very important; however, if I make assumptions about how someone feels, that could rob them of a potential opportunity, or it could also give them a new opportunity depending on the nature of my assumption.

Either way, their capacity to act (agency) remains intact.

The very important phrasing, in my estimation, can lead people to neurotically question their feelings less they "rob" someone of the capacity to act. In reality, a more lax quip such as "assuming makes an ass out of u-&-me" gives people more breathing room for their lapses in judgement.