Paging the @FediTips account, because I'd also be interested in the answer!
There is no direct way to subscribe to a post made by someone else. Nearest I could suggest is using the reminder bot to make you check back on the conversation in the future:
https://fedi.tips/is-there-a-reminder-bot-for-mastodon-and-the-fediverse
Thank you. That is a shame. If I knew how to make a feature request, then I would. It could be good, a little option 'show alerts for replies to favourites/boosts'.
Thanks for the reminder bot tip
@RobertoArchimboldi Indeed. It's also a reason people explicitly cited to me for leaving mastodon, because they wanted to keep up with conversations, but couldn't.
@philippsteinkrueger @FediTips I think that the difficulty of keeping track of conversations is annoying. It also encourages grandstanding, something that I am guilty of. I end up shouting rather than thinking because that is what the medium rewards.
Still, see my post of yesterday, it is at least a bit on us, the users, to make it work and since I consciously tried to shout less and think more, I have had a better time. It could also coincide with an uptick in use because of the latest exodus from twitter
It's been suggested, if you're comfortable using Github you can give it a thumbs up at https://github.com/mastodon/mastodon/issues/7969