Long story short: there are jerks. It's an online game, and unlike many online games it's painfully obvious when someone on the team is having a bad game. In an online shooter, you tend to notice your teammates less, and there's no penalty to just leaving the server if you're doing poorly. In Dota 2, you'll be spending ~40 minutes or more with your team, and it becomes really obvious when someone isn't pulling their weight.
If you're feeling discouraged, try the following (in order):
* Finish the tutorial. You can always click the "Go on a training mission" to resume the tutorial. I
strongly recommend playing every part of the tutorial at least once - it's a big help.
* Mute the offending players. Remember that guide I linked above? There's a reason the location and use of the Mute button is one of the first things it teaches you.
Players can still report you, but frankly, it's not a huge deal if you get reported. Admittedly this won't help you improve, but practice is the best way to get better, and it's hard to practice when people are bitching at you!
* Play co-op matches. Co-op bot matches pit 5 human players versus 5 bots. If you pick Medium difficulty or lower, you'll probably be able to steamroll the bots. It's a great way to learn how to play if you really want human teammates.
* Play a few solo matches versus bots. There are a bunch of advantages here (you can customize your opponent's skill level, you can leave at any time without penalty). Of course, you should have gone through 5 bot matches as part of the tutorial, but this is a nice resource if you've finished the tutorial and want a slightly higher challenge.