This nature is true on a spiritual level as well, yet we expect it to be the opposite. If our avodah is easy and everything goes smoothly, we think we're doing great. We need to know to look deeper and work harder. If we aren't hitting any obstacles, perhaps we aren't working on the right things. Perhaps our hearts just aren't in it, and after all it's our hearts that HaShem wants. (רחמנא ליבא בעי)
The Komarna Rebbe (Netiv Mitzwotecha, Emunah 4:5) speaks at some length about how many challengers and accusers come against the Tzaddik. Rebbe Nachman explains that these challengers protect a Tzaddik from judgements or opposition in heaven. The Komarna takes a slightly different tack explaining how the challenges themselves educate a Tzaddik about what he needs to do, and in fact prooves the righteousness of the Tzaddik.
He explains that if you don't experience any opposition, it's a sign that you aren't accomplishing very much. Those people who think they are Tzaddikim, but who never encounter strife are in fact fooling themselves.
To me, who generally avoids conflict, and who clearly doesn't do anything big enough to offend anybody, how do I take these words of Torah? Do I take it as a lesson that I need to be more intrusive and offensive? That I need to upset those around me more?
No. To me, I apply this idea on a smaller level. If my yetzer hara isn't giving me trouble, if life isn't throwing any challenges my way, and everything is going swimmingly, then I need to start to worry. But, if my yetzer is pushing all of my buttons, and small challenges are trying to crush me under their net-weight by teaming up against me, then I can rest assured I'm still making progress.
The goal isn't to find a way to feel comfortable where you are, actually it's the opposite. Just as the best exercize is happening when you're really feeling it, so too, the most spiritual growth is taking place when you feel the effort of it.
To grow, to build, you have to give it your all, you have to feel it.