'That's' is short for 'that is.' Here you are telling someone 'because of this' or 'therefore.'

'That's'是'that is'的缩写形似，在这里是告诉别人'因为如此'或者'因此'。

Here are some examples: 这里有一些例子： 

"That's why people admire you."

  这就是人们钦佩你的原因。

"That's why she appears so happy."

 这就是为什么她看起来那么高兴的原因。

"That's why babies crawl before they can walk."

  这就是为什么婴儿在会走路之前先爬行的原因。

"That's why Pam cries at sad movies."

 这就是为什么帕姆会在悲伤的电影中哭泣的原因。

"That's why you fail to understand."

  这就是你没有理解的原因。

"That is why you help out people in need."

 这就是你帮助有需要的人的原因。

"That is why you try and include everyone."

 这就是你为什么要试着包含每个人的原因。

"That is why you lock your doors when you leave home."

 这就是当你离开家的时候要锁门的原因。

"That is why she smiles when you walk by."

 这就是为什么当你走过的时候她会微笑的原因。

"That is why you use it for emergencies."

 这就是为什么你在紧急情况下使用它的原因。