To me, "no promises" can have two meanings. But this is only in my opinion.
First, it could mean that a person does not want to promise anything in case they break them. So when you say to someone that you don't want any promises form him, it means that you don't wanna be disappointed in him.
Second, it could simply mean that it may not happen but hopes it may happen. Like, a girl would say to her boyfriend, "I may wanna sleep with you tonight. But no promises."