We knew Spike had a crush on Rarity ever since "Friendship is Magic Part 1", but it isn't confirmed until he tells Twilight Sparkle and Pinkie Pie in "Green isn't your color". Did the creators have Spike have a crush on Rarity just so that they can show little boy crushes, or is there something symbolic about it? I think it is symbolic.
Rarity represents the element of generosity, and we learn that greed causes dragons to grow. In "Secret of my Excess", Spike became greedy and terrorized Ponyville. He continued to grow in size and greed as he continued to collect. When he kidnapped Rarity, he noticed the fire ruby he gave to her. Not knowing the dragon was Spike, she told him who gave it to her, and he shrank back to his normal size.
After watching "Secret of my Excess", I started seeing Spike's crush as something deeper. Spike could have had a crush on any pony besides Rarity, but why did the creators choose Rarity? I think it was because even though Spike is a dragon, he is generous. I believe his crush on Rarity symbolizes his willingness to go against his own instincts and be generous instead of greedy.