I carry a support phone for my company. It usually rings at inconvenient times, like at a restaurant or at 3AM in the morning. It has a distinctive ring, I think it's the default one on that phone.
The people on the other end are looking for solutions to their problems, and are usually facing a telco company that could be losing money or don't have a phone service for their two to twelve million subscribers. There is a bit of pressure involved and calls from a CEO or head of some department are pretty common.
On a walk to work the other day, I went by a bus stop and someone else's phone went off and they had the same ring.
Late nights, slogging through source code, looking at customer configurations, being tired to the point of hallucination all came back to me and ruined my morning.
Pavlov knew what he was doing. It doesn't mean we have to like it, but it probably means that we respond this way to everything. Think about the last time you were in a bad mood and see if there is anything that triggered it.