I would say that more desire leads to greater ease of achieving erection.
I’m sure you have heard it said many times before in some fashion, “The greater percentage of one’s sexual responsiveness is in the brain.”
Baring any uncontrollable physiological condition, the average guy should be able to get it up whenever he wants to, the operative word here being “want”.
If you are turned on at the moment and want to screw then there should be nothing to prevent an erection, regardless of how many times you had done it prior.
I have had occasion in the past, and have heard of others reporting the same thing, that when I was involved in a new relationship, having sex 3 times a day was possible.
Later on the frequency tapers off on account of complacency. But that is only a matter of mental attitude.
I’ve slept with someone and had sex before going to sleep, in the middle of the night, and then when waking up the next morning.
I’ve even had sex with different partners within a few hours of each other. Never did I give any thought to whether or not I could get it up on account of frequency.
I wanted to do it so I did it.
I cannot recall a time that I ever had difficulty getting it up if I genuinely wanted to fuck. There where times when I had the opportunity and did not want to, or had some anxiety about the situation, and could not get hard, but that was rare.