Why does my chopped beef come out too tough? Whenever I try cooking chopped beef for stews or curries, I can't seem to get it right: It always comes out too tough and tastes like dog food.

I usually get beef labeled "for stir fry" or "for stew". It comes pre-sliced, and it seems like it's pretty good quality.

I pat it with paper towels to get excess liquid off, season with salt and pepper (like one would with a steak), put it in a pan over medium high heat with oil until it turns brown. At the end of this, I try tasting and it comes out inedible.

Is there some trick to cooking beef like this - is the important thing here how long it's cooked, or the seasoning, or how lean the beef is? Or is it a combination these?

In response to comments: I live in the southern US. I buy meat from the supermarket; the quality has been very good for steaks and burger patties, but with stew beef I am somehow having this issue. I haven't tried just buying a good steak and chopping it up myself, I'm asking here about beef that is already chopped in the package.
As Raditz_35 mentioned in his comment you prepare the meat in different way than you want to. Yes, in curry and stew/gulash you use low quality (it's not actually low quality per se, it's just more dense and more chewy onto itself) meat. And then you STEW the meat for few hours.

You try to fry it for few minutes. 

If you want to have a stir-fry you need to cut the meat even thinner. Think paper thin. Then you fry on very high heat with high smoking point fat.
If you want to make a stew/curry you need to follow the recipe. Almost all recipes I've encountered counted the time of in hours or stated "simmer until beef is tender". 

Browning the meat on pan before stewing is important part as it keep all moisture inside the meat so it' not come out dry.