Hello Friends,
Many reasons. Firstly oral or dental related. Improper brushing, not cleaning your tongue, decayed tooth/teeth, sinus problems etc.Most of these can be solved by visiting a dentist. Proper scaling(cleaning of teeth), maintainence and fillings, if required, will take care of most of the problems.
if then too the smell doesn't go, you should go to your physician it can be anything from diabetes to liver problems to even stomach related. Hope u catch the real problem.
Thanks In Advance