When talking with teachers I did hear reasons for not sharing:
Information is valuable, I could make money for it.
People do use my materials and they do not share theirs. I ‘ll scratch your back and you ‘ll scratch mine.
I did make it, and it was not easy, I want something in return.
People get lazy, they only copy and paste. They should make their own lessons.
Why should I put my lessons on the internet? To much work for me.
OK, some people do get money for information. But information used in classes is common knowledge. “… Content is easily duplicated and has no value. What is valuable, however, is that which can’t be duplicated without additional input costs: …” (George Siemens).
If you do share your things add a note you want comments, and feedback.
Add a note you want something in return. Invite people to share their materials with you. In Wikiwijs and websites like that users may add appreciation and feedback.
The maker of shared materials has a right of recognition. Use Creative Commons to make clear you appreciate recognition.
Sharing is a sign of appreciation. Your lessons are valued when colleagues do want to use your lessons. Not all teachers are good at making educational materials.
( This text is a translation of a Dutch text used in discussing sharing.)