I find dressage the basis for any style of riding. If you want to jump you HAVE to have the basics of dressage at the very least.
If you are riding western you still use the foundations of dressage in regards to leg yielding etc
I do at times find flatwork very boring but its also exhilirating when your horse does a beautiful extended trot for the first time and you feel the power surging forwards underneath you. Or any new move that you have been working on that has finally come together and it feels so good
The challenge and discipline in dressage is the thing I like the most. You can ALWAYS be learning and no one will EVER know everything about riding.
I gotta admit I much prefer jumping or something a little more 'upbeat' :)