I love Pinkie Pie with all my heart, I really do. She is fun, random, energetic, and puts the happiness of others before her own. That is not to say she is perfect. In the episode A Friend in Deed we get the chance to see Pinkie Pie at her worst, and I don't mean cupcakes. Unfortunately, Pinkie Pie is the kind of pony who needs every pony to like her. Remember her reaction to Twilight Sparkles moving to Ponyville? The truth of the world is that not everyone is friends with everyone. Equestria is no different. Poor Pinkie Pie has to learn this the hard way.
Personally, I would have let Grumpy's indifference slide, but not Pinkie Pie. She becomes so obsessed with gaining his friendship that she drives him to hate her. In the end, as always, they make up. But up until that point is a very important lesson, and one that I think Pinkie needed to learn much earlier in life. It goes far beyond the fact that not everyone in life will be your friend. I think the true lesson is that you cannot force yourself onto others. It comes across as desperate, for one. Also, your self worth should not be based upon those who do not like you, but those who do. Granted, if you have no friends then you probably do need to do some re-evaluating, but my point still stands. She has an entire town who loves her. Why should one donkey, who is obviously just an unhappy person, make her question herself?
read more: My Little Monday: A Friend in Deed