诉 pairs the speech radical 讠 with 斥 'to rebuke': speech with grievance in it. Originally it meant bringing a complaint — you 诉'd to an official. The everyday 告诉 'to tell' still carries a drop of that urgency.
诉苦 'pouring out bitterness' is telling your troubles; 诉说 is to recount at length. In law, 起诉 is to prosecute and 上诉 to appeal — the character never left the courtroom even as it moved into living rooms.
Speech 讠 with a scolding edge 斥 — TELLING what weighs on you.
In 告诉 the second character goes toneless — gàosu — one of many words where Mandarin swallows a tone in fast speech, like English swallowing the 'o' in 'chocolate'.