In Brahma Vindoopanishad-1, there is a verse:
"Mana Eva Manushyaanam
Kaaranam bandha-mokshyaoh
Bandhaaya vishayaasaktam
Muktyai mir-vishayam smritam"
Meaning: For human beings, mind is the cause of bondage and release.Mind attached to objects is the cause of bondage; while the mind devoid of objects is the cause of liberation.This is what sages proclaim.
Our Sadguru says - the poor Sudhama never had any worries. He was always in the state of happiness despite of poverty. Where as demon king Hiranya Kashibu never had peace of mind.
So its all in our mind. When you have high expectations and meet the reality, naturally you tend to become dissappointed where as with lower expectations if you reach bigger heights in life that will make you really happy.
Krishna says in Bagavad-Gita (4.20)
"Tyaktva karma-phalasangam nitya-tripto nirasrayah
Karmany abhipravrtto pi naiva kinchit karoti saha "
Meaning: Abandoning all attachment to the results of his activities, ever satisfied and independent, he performs no fruitive action, although engaged in all kinds of undertakings.
Happiness and difficulties keep coming in our life. Its like a sinu soidal wave. Ups and downs. An intelligent person is never worried about the ups and downs. Thats life. No pill has been yet discovered which will keep you happy forever. But if you are intelligent enough then you know what will keep you happy all the time. There is a Sanskrit Bhajan Which goes this way. Nama Smarana Sukham ..Narayana .........