Life goals and dreams are hard because of their innate complexity to achieve them. It's easy to state "I want to start a family" or "I want to become a business owner" or "I want to climb Everest". It's the execution which is hard because that relies on so many variables most of which you don't really control.
Social background, economic background, your health, your personality, your skills and abilities, your awareness of opportunities or lack thereof, ingrained beliefs and biases, education level, culture, current events, family dynamics, upbringing,...
The notion that you're in control of what happens in your life is a convenient belief, not a hard fact. It's only when that belief is shattered by hard reality that you start to understand that life doesn't necessarily follow the script you had in mind when you're young.
The kicker then is understanding that happiness isn't to be found only when you reach the end of the rainbow, when you finally achieve the dream. It's there along the road, between the rubble and troubles that life throws at you. It's found in what you make out of each day you're given in good health.