Rainbow Dash is the one pony who it seems nobody has a neutral opinion about; everyone I've ever seen talk about her either loves her or loathes her, and nobody seems to have any solid reasons why. The arguments are always just general statements, either "I love Rainbow Dash because she's awesome!" or "I despise Rainbow Dash because she's a stuck up jerk (and "jerk" is putting it mildly compared to the names people usually use to describe her)." Nobody ever seems to give any real examples of her awesomeness when calling her awesome, nor examples of her selfishness when they call her selfish. I've come up with a list of all the pros and cons I can think of with her, based on SPECIFIC details throughout the series.
*Saved Rarity using the Sonic Rainboom
*Saved the rest of the mane six from the tornado at Wonderbolt Academy
*Saved the filly from the well.
*Stood up to Gilda.
*Saved Scootaloo when she went over the waterfall.
*Saved Spike and Rarity in Secret of My Excess.
*Saved the baby from rolling off the cliff.
*Admitted that she was an egghead.
*Saved the seniors from the falling balcony.
*Stood up to the bullies who were picking on Fluttershy.
*Used her wings in the Iron Pony competition.
*Let her fame go to her head before Mare Do Well showed up.
*Fluttershy needed to beg her to give Tank a chance.
*Was one of the ponies who tried to upstage Trixie.
*Cheated in the Running of the Leaves (although, in all fairness, so did Applejack)
Now, of these cons, the one I see cited most frequently is the way she acted in the Mare-Do-Well episode, but people always seem to forget that she was dragged into that by the cheering of the crowds. When she saved the filly from the well, she wasn't doing it for attention, she was honestly doing it for the purpose of saving the filly. When the crowd gathered around her outside the well, she even stated that it was no big deal, but they kept cheering for her. Then, with each proceeding act of heroism, the crowds cheered for her more and more, so she gained a real taste for the attention, and that's also why she felt so desperate to get the fame back after she lost it.
This is the classic psychology that comes with your "Fifteen minutes of fame." She's normally pretty popular around town, but not really famous, so now that she ended up getting a sudden and massive increase in popularity she ended up getting drawn in by the thrill of being cheered for; she gained a taste for the popularity that came with being the "flavor of the month." Therefore, because she was enjoying the sudden rise in popularity so much, she just couldn't stand to see it go. Because of Mare-Do-Well, Rainbow went from hero to zero in a matter of days; very few people can endure a drop in popularity that massive and not at least try to reverse it before they hit rock bottom. It's just basic psychology: when you really get a taste for something, you can't stand to lose it, especially if it means losing it all at once.