@huyilin，

Here 了 is used for emphasis - the feelings you have or your statements. So 了 doesn't change the meaning of the sentence if you use it - just emphasizes on what you want to express.

i.e. 我最喜欢你 you can say 我最喜欢你了.

This usage of 了 is used more often with superlatives.

i.e.这几个城市里，我最喜欢北京了。