Confused

What did I do? Beats me. At the end of Chemistry Practical, Dr Chan returned me back my card which I had personally written for him (together with 23 of my classmates) to give during Farewell Assembly. He never said anything and promptly went back to the preparation room.

What on earth did I write that ticks him off? I thought I wrote my heartfelt gratitude...really confused. Did I write anything wrong? I am going back to school tomorrow.