User blog comment:CNBA3/Fairy Tail 411 Review/@comment-9595671-20141201102747/@comment-7418318-20141201121953

So far Natsu and Wendy only entered DF when they had their own magic plus an external influx, which might mean that they were simply unable to accumulate sufficient quantities of magic power on their own, in addition to a certain anger and/or desperation.

But we know that through training and fighting (and maybe also through growing up) the magic container becomes bigger. Maybe Natsu has just reached the trigger point where he has sufficient magic in his own body to enter Dragon Force without an external magic influx, but the need for the emotional state is still there, so he might not yet be able to freely enter it at will, but only when the situation is dire enough.

With Sting and Rogue, maybe the energy requirement was lowered through their lacrima (along with eliminating the need to be angry enough), but that would also mean that once in DF, they have less energy available to use, thus their DF is much weaker than what we've seen Natsu do.