Quote:
There had to be a better reason why Mephisto wanted to offer Peter to save his Aunt's life and make his identity a secret again then just wanting his marriage.
No there doesn't. Mephisto was a deux ex machina, nothing more than a plot point to get Spider-Man from married to unmarried. To modify a phrase from Marvel, It's magic, there doesn't have to be a reason.
Now, it's possible that we are going to see some "better reason", but I doubt it. Marvel is going to look forward and not mention it again.