Could be a number of things. I can only relate what we see here in the U.S., but I have to assume some similarities. First would be pear psylla... a small bug that produces a substance known here as honeydew which perpetuates fungal growth that can keep young pears from setting. Secondly, any number of fungal infections, such as Pear scab, Fire Blight and others. These, however, usually produce symptoms seen in the foilage, such as yellow or black spots and these types of infections mostly affect the entire tree adversley and noticeably. It could also be that you have a super abundance of pears this year. This happens from time to time and they tend to crowd each other out with many of them falling. If there are several pears all located at one place on the branch try pulling a few of them off to thin them
The other problems will need to be corrected with fungicides and bug killers. Your local garden supply store should have recommendations... Best of luck!