I've stumbled upon your daily quote and must dissect it and rewrite it in proper form below.
It's easier to pretend you're happy than to explain why you are not.
It's is a conjunction of it and is, while you're is a conjunction of you and are.
The word than us used as a comparative conjunction, but the word then is useful for everything else.
The quote can have many interpretations from my perspective. It can mean that pretending to be one way is easier than explaining why you are not the opposite way. If a person is happy, for instance, then all is well in pretend world and people do not inquire further. However, when a person is not happy, it takes more effort to explain the reasoning for the unhappiness by soul searching, listing reasons, and describing emotions. Happiness is a state of mind that is relative to each perceiver.