There is one sense in which the teacher should be predictable: well-prepared lesson plans, punctuality, grading standards, cheerfulness, orderly classroom, etc.
But there is another sense in which a teacher should be equally unpredictable: timely appropriate humor, relevant rabbit trails, impactful stories, and impromptu displays of beauty: art, music, and poetry.
A surprising piece of art, a beautiful poem or even a clichéd joke about polynomials (Rats FOILED again!) might be the inspiration, wonder, or comic relief a student needs to remain curiously and wonderfully engaged in the same ole’ same ole’.