I've sat out several dives just this last year.
1. Galapagos I sat out 2 dives on that trip First time because I was tired & cold from fighting currents & we weren't seeing a lot & the second time because my drysuit neck seal rolled & caused a full suit flood in 50 degree water. I called the dive after 35 minutes because of hypothermia. The next dive (last one for the trip) was to be 20 min. after we got back on board. I was still freezing & figured I did not have the time to change undergarments.
2. Peacock Springs, FL- a couple of days before going through my Full Cave class. A guy & I going through the class together, arrived a couple days early & decided to do some intro level dives. We had a heck of a time trying to do the first dive. 1. the instructor took the keys to his truck (he was riding with another student- we were to drive his truck). After a long frantic search turned up no keys, we called the instructor & he returned the keys (strike 1). 2. When we got to Peacock Springs, my buddy's fin buckle broke as he jumped in. I retrieved the parts & he got it back together (strike 2). 3. the second time he jumped in, his wing started leaking air very badly at the inflator hose connection. I tried to re- screw it back on 3- 4 times with no success. It was too tight between him & his tanks to get my hands in there & to do any good (stirke 3- You're out!). We decided it just wasn't to be our day.
3. The day after our Full Cave course was over, I sat out the second dive. After 2 days of 3-4 dives per day doing Intro level dives, 4 days of going through Full Cave (1200 min bottom time during those 4 days) & one dive in a high flow cave that morning, I was just completely worn out for the day. I did do a dive the next day (last dive of the trip).
4. Called a dive 10 minutes into it. I was helping a friend with a Adv. Nitrox/ Deco course he was struggling with. During the dive (he was leading) I gave him an Out of Air Signal with my light (a drill he knew was going to happen). His attention was elsewhere & not on his team (me). After chasing him a bit, signaling, I stopped & went "unresponsive". He finally came back after a few minutes & I had "drowned". He came with such speed & hit me so hard (no control) that I had to move to keep from getting completely clobbered. Because his control & awareness were not where they needed to be, I called the dive at that point. I drove home the point of his inattention when I told him to call our instructor & my parents to tell them that he had "killed" me because of his inattention. That really woke him up.
5. New Year's Eve midnight dive at the local quarry- Severe thunderstorm rolled through at that time
6. New Years day- Sinuses were in bad shape due to the weather changes from the storms the night before & I just didn't feel like doing a solo dive that morning
That was just this last year, there is nothing wrong with sitting out a dive if you're not up to it, not feeling good or even if it just doesn't feel right (for me the 3 strikes & you're out). You can always dive another day. My tech instructor has told me time & again, "if even a fart doesn't feel right, don't do the dive". Meaning listen to yourself & your gut feeling,.... it is usually right.