I have a ds aged 23 months. He has got a very extensive vocab (not bragging but relevant) and picks up new words pretty much as soon as he hears them. But he's never said 'I love you' or anything like that. If I drop them into conversation he'll never attempt to repeat them back even though he'll repeat virtually everything else. It almost seems deliberate! In our house we say we love each other a lot, so I'm surprised he hasn't picked it up yet. I've explained what it means(as much as you can!) but no joy. I know he does love me - he's very affectionate, but isn't one for kisses either. He does like cuddles, albeit short ones (he's always on the go).
I'm interested in hearing when other toddlers have first said this to a parent. I wonder if he's refusing to say it because I always say it to him and dh before bed or before I leave the house, so maybe he associates it with me leaving.