Sorry, but now I am the one not following you 😄 …
What I was trying to say is that when you formulate a decision you always have an associated expectation, be it good, bad, or neutral (for inconsequential decisions that do not matter for this discussion). For the sake of "happiness", let's concentrate on the long-term decisions with good expectations. For example "I will start playing piano because that will give me some peace of mind", or "starting a family and having a kid, will bring me joy" (I am avoiding monetary examples, because rarely is wealth equated with happiness; and I also do not like to say "contentment" because to me contentment is to happiness what MacDonalds is to backyard grilling - cheap substitutes).
What I wanted to convey is that objectively, it is highly unlikely that your piano learning will be a blissful journey, or that your kid would give you nothing but good positive experiences (if so, I want to trade in my 4 years old 😀 ). However, in the eyes of the beholder and for the same exact piano or parenting experiences, some people will tend to value the positive and forget the negatives ("shoot… Twinkle little star is really difficult, but I am enjoying it and after 2 years I will able to pull it off"; "what is the importance of a boatload of diapers and tantrums, compared with one genuine smile from my kid?") and be happy, while some others will focus on the negatives. This is what I called adjusting the reality and not the expectations. And the solution for that is not to decrease your expectations ("well, as long as my kid does not stab me in my sleep for inheritance, I will be happy"). If you study psychology of happiness (and let's not get bogged down with "what is happiness"? Let's define it as what each people perceives it to be, when asked "rate your happiness in a scale of 1 to 10", or otherwise we will discuss in circles), you will see that people that rate themselves as "happy" are the ones that have long-term expectations or dreams (some very down to earth, some really delusional in their unrealistic hopes), while the ones that rate themselves as "not happy" are usually less capable of formulating positive expectations.
Was I more clear, this time? Maybe not…