One of the problems with making decisions rationally is that it's conceptually impossible. Behavioral economist Dan Ariely highlights this by examining the decision-making process of someone faced with two coffee shops across the street from each other, one featuring handcrafted roasts and the other a standard chain where the coffee is $1.75 cheaper:
What you should do (if you wanted to be rational about it) is consider all of the things that you could buy with that $1.75, now as well as in the future, and decide to buy the expensive coffee only if the difference between the two coffees is more valuable than all of those other possibilities. But of course this computation would take hours, it is incredibly complex, and who even knows all the possible options to consider? [The Psychology of Money and Habits]
If you spend some time thinking on the factors that influence the direction of any decision, what you find is that every factor somewhere on a scale from high potential for rational analysis to low potential for rational analysis. Consider, for example, that a puppy's markings have a very low potential for rational analysis when deciding between one of two puppies to take home.
The problem is that even factors like price that fall relatively high compared to puppy spots are still far from being accurate tools for rational decision making. In this case it's because we can't help but make decisions based on context, relying on a memory that's spotty at best to judge the relative value of any monetary amount at any given time (see: How relativity affects every decision we make: an experiment in making $20K worth more than $20K). This is just one rule of human decision-making among a host of others.
Even if one were to accurately measure the various potentials for rational analysis of every factor at hand (impossible), one would then have to accurately compare factors within the overwhelming matrix of results (also impossible).
Our entire complex of heuristics and cognitive shortcuts exists entirely because being rational is simply far too difficult (see: this list of all the ways you could be completely wrong about everything).