Head over heels in love
Definition 1 of 1
Pro Tip 1/3Emotional Intensity
This phrase emphasizes a very intense and passionate emotional experience. 
When she talks about her partner, you can tell she's head over heels in love.
Pro Tip 2/3Not Literal
The phrase is metaphorical; it depicts love as if it turns one's world upside down. 
They're head over heels in love, spending every moment together.
Pro Tip 3/3Informal Expression
It's a casual, conversational phrase, not typically used in formal contexts. 
I'm head over heels in love, and I can't concentrate on my work!