How can I avoid eating some sweets in the morning? What's the reason for it?
The best way to avoid eating things you regret is to not have them around at all. If you've got sweets hanging around, get rid of them. Another great tool is to plan your food for the entire day at least one day before. It's amazing how much easier it is to "eat right" when you've already got everything mapped out in advance.
In addition to the comments alread made, it could also be a simple matter of you having programmed your brain to want sweets in the morning. I used to be the same when it came to a beer... Every Friday on the way back home from work I found I had intense cravings for one. So I changed my routine, I drove home a different way so I didn't go by the store & worked on breaking the mental weaknesses.
Give it a try.. Leave something out for you to eat as soon as you wake up. Break your routine.
It may just be that as you've always done it, you're acting on a subconsciously competent level and it's a trigger reflex
(Jeez! I can really jabber on once I get going!!!)