Question: Does anybody know why Sabo ate the Mera Mera fruit?

![enter image description here](https://i.stack.imgur.com/7y0Dx.png)

I thought Luffy really wanted this fruit because it was the devil's fruit that his brother Ace ate.

Sabo took Luffy's spot in the Colosseum to get the fruit for him.

Answer: 
Sabo never intended to get the fruit for Luffy. When he showed up in this arc, his first words were 

"I'm not letting you have the Mera Mera fruit, Strawhat Luffy!"

Sabo always intended to eat it, and Luffy was fine with that. Luffy obviously can't eat it himself, he just wanted someone he liked to have it. He left the colosseum very happy, fully understanding that Sabo was going to eat it. It didn't come out of nowhere at all.
Short story: Sabo wanted to eat the Mera Mera fruit because he, as Luffy's and Ace's "brother", to inherit his will(have his spirit live on).

Long story: He ate it because, before the final, he was doing it, because he said. wanted to inherit Ace's will. Well, him and Luffy, according to this statement : "We are the ones who will inherit his will!" Besides, Luffy can't obviously eat it, and who better to come along to eat Ace's fruit, then his second brother Sabo!? Seriously, once Luffy fully realized that Sabo wasn't killed, he would obviously let him have the fruit, since he is Luffy and Ace's best friend/"brother".