The main reason why Dumbledore cannot fight Grindelwald is he is in love with him. This is not something expected. A lot of Harry Potter fans were shocked with the revelation but others say that they saw it coming because of how Dumbledore was never portrayed to have loved anyone before. Dumbledore is known to be wise but he has always loved to be in power.
This has allowed him to fall in love with someone whom he sees as powerful which is Grindelwald. It is possible that he may have wished to be like Grindelwald but he decided to stay away from the Dark Forces. He is still known to be one of the greatest wizards who ever lived.