Do people always decide to do things that they believe will provide them the maximum amount of positive feelings in their conscious life? Is there any scientific study or theory which supports the following claim? - People always decide to act in accordance with things they believe will guarantee them the maximum amount of positive feelings in their conscious life.

Are there any other variables that play a role in the person's decision-making?

(*Note please that I don't have a background in psychology.)
I would say that the more compelling scientific evidence would suggest the opposite.

Daniel Kahnemann got a Nobel Prize for behavioural economics, part of which discussed that the default decision making is not a rational, thought through process.  His book Thinking Fast and Slow discusses it quite well.

Dan Ariely's book Predictably Irrational discusses similar concepts, and covers some cognitive biases - specific examples of how people make decisions other than following rational decision making process.

Both of these books are readily accessible for people without a Psychology background.

One of the books suggested a way to reduce the impact of this tendency: draft out the framework for the decision making process before going into the decision environment.

I'd suggest that more decisions are made from the aspect of minimising negative feelings in the short to medium term.