How to select a Master's thesis topic if your advisor won't suggest one?
One approach is follows:


Find one or two good recent PhD theses in your chosen area. Read these thoroughly. As you read, write down every question that pops into your mind, write down every time the author states that something is left for future work or needs further investigation. 
This will have a dual effect. Firstly, you will get a good introduction to a particular research field. Secondly, you will have a bunch of questions that need to be investigated.
Prune trivial questions, non-sensible ones, etc. 
Next, organize your questions thematically and see whether you can find a common thread to these questions, something that could form the basis of your research. 
Remove any questions that do not fit will with the others.
Based on the remaining questions, formulate a couple of coherent questions that your research could address.
Write a plan to address these questions.
Work through plan.
Write thesis.