"因为他们很开心" 可以用英文表示为 "because they are very happy"。

这句话的来源是英语中的一种常见表达方式，在日常会话中经常使用。

例句:

"Why are they laughing?" "Because they are very happy." (为什么他们在笑？因为他们很开心。)
"Why did you decide to go on vacation?" "Because we are very happy and want to relax." (为什么你决定去度假？因为我们很开心，想要放松。)
"Why did you buy a new car?" "Because we are very happy and can afford it." (为什么你买了一辆新车？因为我们很开心，并且有能力负担得起。)