Sufficient Reason In case I haven't dumbed it down enough;
What is a sufficient reason? — Posty McPostface
The problem with answering this question in any straightforward way is that what kind of thing counts as a sufficient reason can be cashed out in a few different ways. To take Leibniz again as an exemplar, for him, the sufficient reason for any one thing was, in a certain sense, the entire universe. For Leibniz, one cannot think the sufficient reason for anyone one thing without including the entirity of the universe within its ambit. In fact, not just one universe, but
multiple worlds, along with certian logical relations between them, were necessary to properly cash out the PSR. The thing is, this is quite clearly not the only way to do so (and it is often said that Leibniz ultimately fudged the whole enterprise right at the end with the God stuff, and didn't quite fulfill even his own strictures on the PSR).
Deleuze, for instance, has a reworked understanding of the PSR that dispenses with the many worlds, along with the God, while at the same time insisting on its non-causal, transcendental operation (and there's lots of gaps to fill here with respect to just what that means). So, long story short, what counts as 'a' sufficient reason depends on who you ask, and it won't be some simple answer like 'because x'. As Deleuze said of Leibniz, his pursuit of the PSR required a 'crazy invention of concepts', each interlocking in various ways in order to really do the work of the PSR. The PSR on its own simply says: there are reasons for why things are as they are and not otherwise. Exactly how this is put to work, is the work of philosophy.