You can talk about mentality/bottle etc. but as far as the Djokovic/Murray match up goes, a stat as simple as first serve % is massively important. Murray's second serve has always been a major weakness and it's something he's unlikely to fix at this stage in his career. When Murray's first serve % dips below 50% it's almost inevitable that he'll lose a lot of service games as he'll lose the majority of points on his second serve.
Agreed although as I said in a previous comment I do think that Murray is, at his best, a superior grass court player to Djokovic. I really thought that Murray might win consecutive Wimbledons after he won in 2013 but he simply hasn't been in top form the last two years.
Just checked their stats on Wikipedia and Murray does indeed have a better grass court record than Djokovic (84% to 82%). Of course the sample is much smaller as there are far fewer grass court tournaments but it's interesting to see that grass is by far Murray's best surface.