The first thing that makes these clubs intense rivals is their histories.
Arsenal outshines Chelsea in terms of trophies won with 13 First Division titles, 10 FA Cups, two League Cups and once the Champions League runners-up.
Chelsea, on the other hand, has four First Division titles, six FA Cups, four League Cups and are also one-time Champions League runners-up.
Although the difference between the two may seem pretty large with Arsenal casting quite a shadow over their West London neighbors, if you look only at the past decade, the trophy count gets much more competitive.
Since 2000, Chelsea has three League titles, compared to Arsenal's two. The Blues have four FA Cups, compared to the Gunners' three. Both have just missed out on the coveted Champions League trophy just once.
Looking at those stats, it's easy to see why the rivalry has heated up in the last few years. Both clubs know that one more trophy win on either side would make eclipsing the other that much more difficult.
That is, after all, what local rivalries are all about.