A doctor said this and it was such a good metaphor: imagine that you have a 3000 year old machine (which no one understands anymore) with a bunch of cogs and gears, and you’re looking down at it from above. You’re trying to fix a gear that is slightly out of alignment further down in the machine, you can’t really tell which one it is but you know roughly which area, and to fix it you drop a rock into the machine and watch it fall down.
That’s what we’re doing with SSRIs. We’re dropping a rock that manipulates our serotonin, which gives a bunch of effects but not the one we’re after. The one we’re after is somewhere down the line. It’s affected by processes that are affected by the serotonin somehow. We’re not exactly sure which one it is, but we know that if we drop the rock in there it will make the gears align sooner or later.
Which is why it takes time, and why it has some odd effects on people sometimes.
edit: added a detail I missed, to make it clearer.