Rebbe Yakov says that there is no reward for Mitzvos in this world, because there is no place in the Torah where it says "you will live long" more then kibud av vaaim and sheluach hakan. So if the father asks his son to go up the tree and do sheluach hakan and on the way down he falls and dies, where is the long life? So you must say that the long life is in the next world, not this world. (Keddushin 39b, the end of Chullin)
So the Talmud asks, maybe such a story never happened? And it answers that Rebbe Yakov saw it happen. But maybe the boy was thinking of Avoda Zara in which case it's just as bad as doing it? So Rabbe Yakov answers, "that's exactly the point!" Why didn't the reward of the Mitzva save him from thinking of Avoda Zara?
So the Talmud continues, "didn't Rav Elozor say that if you go do a Mitzva you will not be harmed both the way there and the way back? And it answers that it was a bad ladder, where the harm is very likely, and in this case you're on your own, like Shmuel was scared to go to Shaul even though G-d told him to go.
Said Rav Yosef, "if Acher would have only learned the posuk like Rebbe Yakov his grandson did he would never have gone OTD. Now what did Acher see? Some say he saw the exact same story as Rebbe Yakov, and some say he saw a pig playing with the tongue of Rebbe Chutzpis the translator in the sand. So he said "is this what happens to the tongue that gave out so much Torah? And he went OTD.
So now the question is this. Rebbe Yakov obviously has to deal with the problem of 'bad ladder' just like he has to deal with the sin of Avoda Zara. In other words "what is your proof? It was a bad ladder! So you have to say "that's exactly the point" if Mitzva had a reward it should have saved him from the bad ladder. So the question is, why do we need both the bad ladder and the sin of Avoda Zara? And you can't tell me that you need both in order to drop dead, because Shmuel only had the bad ladder and still he was scared.
So you must answer that even if a Mitzva protected you from a bad ladder it would not necessarily protect you from sin, so Rebbe Yakov has to tell you that yes, the reward of a Mitzva should even protect you from sin (If we had the reward in this world). But there is no reward in this world.
The Talmud in Sota 21 also holds like Rebbe Yakov, that a Mitzva will protect you from harm, but it will not protect you from doing a sin even while you're doing the Mitzva. But Torah will protect you from sin only while you're learning. When you're not learning it will only protect you from harm. So the question is, how does Torah protect you from the yeatzer hora while you're learning? I though there is no reward in this world? And the answer is that it's not the "reward" of Torah that's protecting you, it's the "fire" of Torah, like it says in Keddushin 30b, that Torah is like fire and like water, it melts metal like fire, and it smashes stones like water.
The truth is that you cannot have the reward of Mitzvos in this world, because the reward of doing Mitzvos (Because G-d told you to do it) is so great that it will not let you sin at all, and it will take away your free choice. I think Rabbi Akiva Tatz says the same thing.
You look back at your life and you see all the Mitzvos you did, all the Sukkos you built, all the dancing you did on Simchas Torah, all the Paysach cleaning you did at home, all the davening you did on Rosh Hashana, and you look at your life, and you say "where did all my Mitzvos go? Why is my life just as rotten as ever? Why did I need all this? For what? And then you start having doubts....... That's midlife crisis.