its about expectations and resilience, if you are the sort of person who just expects that everything in life is going to be the best with the best possible outcome then you are easily disappointed when something minor happens and over time become less resilient. If you always look for the worst possible outcome then you are less surprised when things go badly so over time your acceptance / resilience for these things gets stronger and you cope better.
It is far healthier to look at what the best that could happen is and what the worst is and then look at what is the most likely thing to happen and go with that.
so you get your shopping delivered... worst scenario bread is squashed half the items are missing and the subs are rotten. No surprise if you were expecting this, just a 'meh life can be shit sometimes' you have developed the resilience to deal with it, but a total headfuck if you were expecting best scenario leading to outrage and major disappointment as it was totally not expected.
best scenario, bread is fresh fluffy and unsquashed, no subs at all and an un asked for bottle of wine appears randomly in your order and the store says 'never mind, you keep it' Yay life can be good sometimes especially euphoric if you were expecting the worst, totally accepted as normal if you were expecting this as the best, no resilience required or indeed learnt.
Most likely scenario, everything is there except for a couple that have been subbed adequately, bread is a bit squashed at one end but fluffs up with a quick squeeze, you survive with no trauma and life goes on as usual, because you have considered what the worst could be and the best and the actual outcome was fine no major disappointment and no euphoric moment either, your resilience for dealing with minor issues has been built and used in this scenario.
The news is full of stories of extremes of this people with great resilience who suffer major setbacks in life but go on to use that experience to do something strong, refusing to be seen as a 'victim' and also full of people with no resilience who totally melt down at the slightest thing.