on one's soapbox

get / be on one's soapbox (to speak passionately on a subject you feel strongly about; to tell other people strong opinions in an annoying way) — ораторствовать (ирон.), резонерствовать; произносить пламенную речь; речь толкать; ≅ сесть на любимого конька; ≅ завести любимую пластинку; ≅ демагогию разводить; заладить,
разоряться {прост., неодобр.}.

Example 1: He’s always getting on his soapbox about student poverty. — Его хлебом не корми — дай поговорить, как бедстуют студенты.

Example 2: It was that point in the evening when my father got on his soap box and started lecturing us on the evils of the modern world.

Example 3: I kind of get on my soapbox, but I feel a responsibility to comment on these things.

Example 4: You know, I could get up on a soapbox and say we should all be vegans and eat nothing unnatural and so on and so forth, but I don't live that way, so it would be somewhat hypocritical. (The Tricky Business Of Eating Right)

Related vocabulary:
be / go on about sth (to keep talking about someone or something, especially when other people think it is boring)
go on and on about sth

Example 1: She’s always on about her children.

Example 2: Dad got on his soapboat and went on and on about how expensive it was to get an education these days.

harp on about sth (to keep talking or complaining about something in a way that makes people bored or annoyed)

Example: Stop harping on about how ill you feel.

hold forth on sth (to talk for a long time about something that is important to you, often in a way that other people think is boring)

Example: Dave was holding forth on the subject of politics.

A soapbox is a wooden box that people stood on in the past when they were making a speech in public.

See also
[where is your soapbox?]