Random thoughts:
I think that T'Pril loves that casually energized aspect of Sevet, but she also lives under Vulcan society's prejudices, so she reacts much more sternly to him (and T'Pring) to mitigate it. Sarek's predilection towards emotional people made he and Sevet natural friends, and T'Pril has warring ideas of her fondness for emotionality and her own fear of being ostracized, and so tries to stamp it out where she can. Sevet just goes along with it all because he loves her and he's a bit oblivious.
I could see the betrothal of Spock and T'Pring as being attempts by Sarek and T'Pril as an attempt to prove their bona fides with a society that looked down upon them.
I've always wondered why Amanda Grayson, a modern 23rd century woman, would agree to such an outmoded idea of an arranged marriage for her son. I'm thinking she got spooked (by Sarek, probably) about Spock going insane and dying during an unbonded pon farr, so she reluctantly acceded. She probably also figured that since Sarek was divorced (?), if Spock were unhappy, he could ultimately end the marriage.