To rationally make a choice you weigh up all costs and benefits of all possibilities and choose the one with the greatest net benefit. To rationalise a choice you want to make, you choose costs and benefits that lead to your choice seeming like the rational conclusion. Thinking you’re being rational while completely ignoring known costs and benefits that don’t lead to your preferred conclusion is hard to do though. Even slight intelligence leads you to notice things like this happening in your mind.
For most everyday decisions I suggest the ‘solution’ lies in probability estimation. While you might have a set of outcomes you consider possible, their likelihoods are virtually always uncertain. It’s a guessing game, and if you’re guessing, why not guess things that lead to the conclusion you prefer? You might even notice while you’re doing it that your probability estimates are being swayed by the conclusion they’ll lead to, but it doesn’t matter. Within the range where there are no other bases for their positioning, why change your estimates to ones with a less pleasing outcome in the short term? Essentially we slide partiality into the one non-rational part of a rational process.