You will not always feel better afterwards
Most of what people say about running is about the afterwards. It clears your head. You come back steadier than you went out. Somebody at work will tell you they are a different person on the days they go, and they mean it, and none of it is a lie.
Then there is the evening where you do all of it the same way and nothing happens. You get out. You do your half hour. You come back, you shower, and the flatness you carried out of the door is still sitting there when you sit down. No lift. Not even the mild satisfaction you were counting on to make the whole thing worth the bother.
The flat ones never get talked about
They make a poor story. Nobody comes into the kitchen to announce that they went for a run and felt exactly the same afterwards. What gets reported is the good one, the evening that turned a bad week around, and after you have heard enough of those you can end up with the impression that the lift is standard issue and you are the only person it keeps skipping.
It is not standard issue. A great many runs are unremarkable. You go, you come back, the evening carries on as it was going to anyway. That is the ordinary case rather than a fault in the way you are doing it, and if anybody had said so at the start you would have been spared a fair amount of confusion.
What you do with it afterwards
The flat run itself costs you almost nothing. The damage is in the sum you do afterwards, usually around nine in the evening, when you are tired and not at your most generous with yourself. If this is not making me feel better, what am I doing it for. And underneath that, quieter: perhaps it works for other people and not for me.
That conclusion arrives dressed as an observation. It is mostly fatigue with a case to make.
Your mood on any given evening has several authors and the run is only one of them. A conversation that went badly at three in the afternoon will outweigh it. So will a short night, or something waiting for you tomorrow that you have been avoiding. Thirty minutes of moving does not get a veto over the rest of your life, and it was never going to.
How you felt is a poor grade for the session
The work does not ask you for a review. An easy half hour done in a foul mood is the same half hour. Your legs and your heart took the same amount of it either way, and they will go on quietly making use of it while you sit there deciding the evening was a waste.
This matters more than it sounds. If the lift is the reason you go, then every flat evening becomes evidence for stopping, and there are going to be a lot of flat evenings. You are trying to build something that survives ordinary weeks. Ordinary weeks do not hand you a reward at the end of every session.
Something you can actually check
When the feeling does not arrive there is still one thing left on the table: you said you would go out and you went out. That part is verifiable. It does not depend on your mood, or the weather, or whether the run was any good. It is not much of a story to tell anyone, but it is the thing that is genuinely accumulating while you wait for the interesting bits.
Give the feeling a longer window as well. It has a habit of turning up later than you went looking for it: the next morning, or halfway through a Tuesday when running is nowhere near your mind. If you check for it in the ten minutes after a shower, find nothing, and file the evening under failure, you will miss most of what it was going to give you.
Going out without being owed anything
There is a version of this where you go with no particular expectation of being paid in mood. Written down that sounds bleak. In practice it is the kinder arrangement, because a run that does not have to justify itself by improving how you feel never has to be defended on the evenings when it doesn't.
You are allowed to have done something good for yourself and felt nothing much about it. Most of the genuinely useful things you do for yourself are like that. Nobody experiences a surge of anything after brushing their teeth, and nobody takes the absence of it as a reason to stop.
The evening you went out flat and came home flat still counts. Put it on the pile with the others and go again when it is next your turn.
Written to encourage, not to diagnose. If something hurts or feels wrong, please speak to a doctor.