So my boyfriend of 15 months and I were going to a party, and he wanted to buy me a new dress. I told him that I didn't need anything new, and I didn't feel comfortable with him buying me clothes for no reason. He then flipped out and told me "it's not about you, it's about me." When I asked him to elaborate he said that I'm never willing to accept anything from him, and it makes him wonder if I am as serious about marrying him as I think I am...what? Where did he get all that from the fact that I don't want him wasting $$$ on clothes that I don't need?
You are a smart and practical girl, and I'm sure he appreciates that in many other ways, but in this case, he wasn't buying you a dress because he thought you needed one, he was buying it becasue he wanted to show you how much he cared about you and how much he wanted to make you happy.
You rejected his gift with the best of intentions, but you still rejected it, and he's hurt by that. As long as he can afford to buy you something occasionally, and isn't going into debt or spending a stupid amount of money, it's okay. Accept those occasional gifts graciously and thankfully, and be practical in other ways. That's win-win.
You really, really missed the point. He wants to share in your life and for you to share in his. If he wants you to wear a dress he especially likes, why is it that you can't be gracious and go along happily? Whether you think you need the dress or not isn't the point. The point is he wants you in it and wants to make it a gift to you. I strongly suggest you apologize, graciously (notice how often I use that word?) accept his offer, let him buy the dress, thank him graciously, and wear it with style and grace.
He thinks you look beautiful and by rejecting his gift, you denied him the pleasure of him giving you an opportunity to show that he has an amazing woman at his side. Rejection of a gift given in earnest hurts. I know you meant well by trying to have him save his money, but if the dress wasn't super expensive, I would apologize and accept it. Your relationship will be the better off for it.
Pretty much everyone has said it all already. It was a gift, a way for him to show you that he thinks you are beautiful, that you're special, that he loves you and that you're worth it to him. It's a way for him to show his admiration for you, that he's able to buy you pretty things, that he wants to make you happy.
It could have been a dress, it could have been a toy. The actual thing doesn't matter - it was more the emotions behind it.
So for you to reject his gift (and when I say this, I completely understand why you did it), he probably saw it as a rejection of his love and all those things listed above. Next time, just accept it happily. If he really couldn't afford it, I assume he wouldn't have offered.