There are few factors which motivates to answer a question on Musing Platform.

First one is the question itself. Sometime I feel that I know about the question asked and should contribute towards it.

Second factor is the topic. If some topic interest me than I will also do some research to answer that.

Feeling of Help, If it is in your mind that by answering the question you are helping someone then you would be automatically get motivation.

Another factor is encouragement. if your efforts are encouraged in form of comments , replies or upvotes then you will to motivated . 

Then it comes to Upvotes, If someone is getting good upvote , he/she will automatically motivated towards answering the more and more question.