<p>Okay, so truth is that they’re saying that they’ll do a favor for you, so you should be appreciative. And frequently they end up not doing it simply because they forgot to do it. I’m usually just talking about small favors of information. </p>
<p>But it also seems common for people to say that they’ll “do it” just to get you to shut up, or because they want to pretend to be nice/generous (believing that you won’t press them further). Sometimes though, they do tell you to “remind them to do it” (which, IMO, is the nicest thing for them to say). Since sometimes, people who want nothing more to do with you will say that they’ll “do it” but will probably ignore you if you remind them to do it (although they most likely won’t tell you to remind them to “do it”). </p>
<p>Of course, it could be your fault for asking too much to begin with. But it could happen with small information-based favors.</p>