Sentence Ending Particle よ

“I’m telling you”
Sentence + よ

Sentence ending particle よ is used to strengthen a statement, typically by emphasizing that it’s true. 
よ throws your opinion at the listener. It insists that the statement is true. よ statements tend to sound more subjective.
よ tends to grab the listeners attention, so it can be annoying if overused.

Emphasis

よ can be used as emphasis. よ can come across as nice and reassuring, or overbearing and condescending. Be careful not to overuse it. 

• 愛している – I love you
• 愛しているよ – I love you (don’t doubt it)
  
  
• たいへんだ – It’s hard
• たいへんだよ – It’s hard (believe me) (Sounds like the other person doubts the statement)
  
  
• だめだ – It’s bad
• だめだよ – It’s bad (I think you don’t know) (Listener is a child?)

Warnings

よ is almost always used for warning type statements. This can be a general warning or a warning about what you will do. 

Examples

• あぶないよ – It’s dangerous. 
• ころぶよ – You’re going to fall.
• たべたらしぬよ – If you eat it you will die.
• むししたらみせないよ – If you ignore me, I won’t let you see it. 
  
  

Mistakes/Explanations

よ also gets used to correct mistakes; or for explanations (be careful using it like this)

Examples

• 赤ですよ – It’s red (you thought it was green?)
• おわったよ – It’s finished (you thought it wasn’t finished)
• あめでぬれたよ。- It got wet in the rain. (Other person didn’t know)

More Information

There’s actually quite a lot more depth to よ. If you’d like to read more (In Japanese). You can check out this site here. There’s a good 5+ pages all about よ.
It served as a source for some of the explanations here.
The first image on the site is a good representation of よ.