> For a given person, an SSRI may significantly relieve their symptoms, have a mild effect, or have no effect.
In other words, SSRIs do not work for everyone who has depression. For some people, they do; for others, they don't. You and your grandfather apparently are in the first group; but there are also plenty of people in the second.
Now comes the obvious next question: is there a way to tell in advance (i.e., without trying the SSRI on the person and seeing whether it works or doesn't) which people are in each category? No, there isn't. That's the impact of not knowing the mechanism: treatments for illnesses like depression are trial and error. If you find a drug that works for you, great! But many people never do.
In other words, SSRIs do not work for everyone who has depression. For some people, they do; for others, they don't. You and your grandfather apparently are in the first group; but there are also plenty of people in the second.
Now comes the obvious next question: is there a way to tell in advance (i.e., without trying the SSRI on the person and seeing whether it works or doesn't) which people are in each category? No, there isn't. That's the impact of not knowing the mechanism: treatments for illnesses like depression are trial and error. If you find a drug that works for you, great! But many people never do.