दिल लगाना has nothing to do with love! Rather, you can consider that as मन बहलाना.
The meaning of this phrase in Hindi changes according to the usage of it in a sentence. Study this:
उसका वहा पे दिल नही लगता| (Literal translation: her heart is not attached/involved there) - She's not happy there!
So, yes, in a way, you attach your heart with something or someone you like. But it is not always love.
If you want to talk about the intensity of love, there are many ways. One such way is:
He loves her so deeply that he cannot live without her.
For non-native speakers of Hindi, we have this idiom to talk about one's heart being happy doing something or being with someone. That something/one could be animate or inanimate object. दिल (pronounced: dee-l) means heart, and लगाना (pronounced: la-gaa-naa) means attached. Together they translate: heart attached/involved.