"Assertive", pachyderm, not "aggressive". You don't ever want to growl at and bite others, even in one's own mind (as that makes it harder not to respond that way in action)

It is okay to say what you want and what you are going to try to do, set personal boundaries, etc.; it is not okay to do that as a child if it is "frowned upon by the establishment". That's where you end up learning to pretend to be passive but, one's desires must out! So, since you don't learn how to respond in the moment in a way that satisfies you and does not impinge on another, the pressure makes it come out somewhere else, a little too forcefully :-)

I still remember my, "I hope you're enjoying your bourbon and water!" said to my stepmother when she was ordering me around what chores to do while she was. . . enjoying her bourbon and water. Being a teen or 20-something, I missed the crucial point that she had prepared for the party and fixed the meal, etc. so it was my turn to help with the after meal cleanup. She did not ask for my help, demanded it and I did not say, "I would be glad to help but it would appreciate it if you asked me to rather than just assuming or ordering me to?" or anything, just let a slow boil start until I erupted and made a mess of the "actual" problem.