He had just inhaled a single breath from a new atmosphere. (1.6.62)
Boom! That's the way a skilled writer describes falling in love: a new atmosphere, because Jude-in-love is living in a different world from Jude-not-in-love-but-only-interested-in-studying.
This [feeling for Arabella] seemed to care little for his reason or his will" (1.7.6)
Jude is like a lot of us. He falls in love and everything else in his world kind of gets thrown upside down. His past thoughts and dreams are subdued by this one feeling. Unfortunately for him, at this point, that feeling is about Arabella, and we all know how that works out.
But courting was too cooly purposeful to be anything but repugnant to his ideas. (1.7.12)
Jude is an emotional guy. When it comes to love, it's about feeling and action; it's not about planning and calculation. Maybe this is pure, and maybe this is what gets him into trouble. You be the judge.