on (one's) soapbox
Vocalizing one's opinions, beliefs, or ideologies in an impassioned, impromptu manner, often to others' annoyance. Soapboxes (crates in which soap was shipped) were once commonly used as makeshift platforms for such speeches. Once Grandpa got on his soapbox about the local election again, I found an excuse to slip out of the room. Whenever Jim is on his soapbox like this, I just try to tune him out.
on one's soapbox
Expressing one's views passionately or self-importantly, as in Dexter can't resist getting on his soapbox about school expenditures. This expression comes from the literal use of a soapbox as an improvised platform for a speaker, usually outdoors. [Mid-1600s]