天天开心用英语可以说是"be happy every day"或"have a happy day every day". 这个短语的来源可能源自于对生活的积极态度和对每一天的美好期待。例句如下：

I try to be happy every day, no matter what challenges I face.

(我尽量天天开心，不管面对什么挑战。)

She always has a happy day every day, even when things don't go her way.

(即使事情不顺她的意，她总是每天都过得开心。)

I wish you a happy day every day and a wonderful life.

(祝你天天开心，过得美满。)