A life without expectations is a life without aspirations.
Disappointment is a fundamental part of moving onwards in life, just because the feeling is unenjoyable does not make it a bad thing. The idea that one should avoid having any expectations will lead to happiness is a bit misleading, much in the same way you could say "Never try and you will never fail". While the statement is technically correct, the quote "You miss 100% of the shots you don't take" can also be applied.
If you want to do something, you need to be prepared to take in the good with the bad, and ask yourself if you want it enough to go through the task, all things considered.
Negative emotions are every bit as important as positive emotions in driving us forward. Of course, there is a balance to strike, having too many feelings of conflict and frustration is not helpful, but it should also be noted that being overly amibitious or positive can lead to the same result.
There seems to be a strange belief held by many people I've met which dictates that you must love every aspect of what you do. To me, that's not what it comes down to.
For me, it is only when you love something enough to accept it with all the faults and flaws that you can become great at it. It is your ability to overcome any obstacle because you believe it will lead to something better. Understanding and accepting that there will be downside is crucial to being able to progress.
I will say right now that not every part of drawing is fun to me, but I can say confidently that I enjoy drawing, because for all of the moments where I fail, there are moments where I succeed. And with every failure and success, I'm learning a bit more, and moving forward.
Just to be clear, I'm not making this post to dismiss what Shinka was talking about, but rather, made it to show that there are two sides to everything, and the important part is being able to discern when to listen to what.