Why is the meaning of "I feel you"? and when to use it?
"I feel you!" When can we use this sentence?
I understand why you are sad...
The closest and simplest meaning for it is "I understand", and we usually use it when someone is talking about something that has caused her sadness, frustration, etc. By saying "I feel you" we show our sympathy to them.
Here is a sample from a movie.
Now, could you make your own scenario/dialogue using this sentence?
What is the meaning of I feel you?
It's an expression that indicates that you feel somebody's pain or emotions
I understand you or I feel your pain or any other emotion.
Mike: I am so tired.
Tom: I feel you, I haven't slept at all.
"I feel you" meaning
It is slang, and it means I agree/understand/ relate to what you are saying.
For example, friend: " learning English is so hard." You: "I feel you, it is hard for me too."
In conclusion
I feel you is a slang phrase commonly used to respond and agree to what another has said.
I feel you - what does it mean ?
1. It expresses that we share the same feeling, idea or opinion with someone.
2. We express sympathy and understanding for someone.I feel you - Examples.
1. I feel you; I'm starving as well.
2. It must be very difficult to loose a person close to you. I feel you.Synonyms
I understandI agree with youI can relate 
I feel you is another phrase for I understand or I agree with what you said. It is also a slang phrase.
Example
"This is too complicated, I don't think I'm going to finish this project." "I feel you. If you want, we can work on it together!"