Gate's suggests that the way out of this seeming contradiction is by divorcing tapas from results. When we see something lacking in ourselves, we look for a way to fill it, and this is the typical wellspring of desire: we desire a particular result. The trick with tapas is simply to desire the practice, or the process, itself without necessarily linking the process to any result.
On a mundane level, I've seen this at work many times. For example, I don't think its a coincidence that I tweaked my back on the same day that I reached my goal for the year of seeing the ballet bar in the first backbend. I'm not sure if it was the cause, but I know that I probably pushed myself too hard because I was determined to meet my goal for the year. Even if the little injury wasn't a direct result, its timing is too perfect for me not to take it as a warning not to push too hard for some objective goal.
A woman at our studio, whose practice is otherwise very strong, has done the same thing repeatedly. She was determined to do well in the asana competition last summer and pushed herself really hard. As a result, she injured her back a week before the competition and had to withdraw. Recently, she became determined to lock out her kicking leg in Standing Bow. And I think she made it. And at the same time, she otherwise blew her practice to hell, got worn out, and now seems to be taking a break. Again, attachment to a particular result was the enemy.
But, if you become zealous about engaging in the practice itself, its perfectly possible to keep up the desire to practice, and also to stay content. In an odd way, I think that on this level desire and contentment become one. And that's how it feels when the practice is going well.
In an odd way, I think that's how it feels when people start to behave charitably toward others. When you start helping others, you shouldn't be doing it to make yourself feel good. But the act of helping others tends to make the helper feel better, to become content, and that in turn drives the person to want to help more. And again, I think its important to realize here that its the process of helping itself that is satisfying. If you decided to behave selflessly to make yourself feel better, I think you would defeat the goal from the outset.
Taking this even to the next level. I used to torture the nuns in catechism with all sorts of logical puzzles about the nature of God. One I especially liked was my proof that if there was a God, he could not have created the universe. The idea was that creation necessarily involved desire. And that a perfect being could not possible desire anything. Hence, no creation and we don't exist (or God the creator doesn't). Pretty clever for a ten year old. But I think Gates here may show the way out of this conundrum, by suggesting that there are ways that desire and contentment can go hand in hand. (Please don't tell the nuns.)