Love looks not with the eyes, but with the mind,
And therefore is winged Cupid painted blind.
- Shakespeare
This quote means that, when we're in love, we don't see it with our eyes but with our minds (imagination). That is why, the God of love, Cupid, is painted blind.
All this while Dev was trying to figure out what that unknown feeling was. The answer for his restlessness, sleeplessness & other alien feelings were answered today. When Dev closed his eyes & imagined, he saw Sona! Finally Cupid's arrow struck him. ❤️❤️
P.S: This is the literal translation of the quote & I've compared it with Dev's current situation. There are other ways this quote can be interpreted.