I'm a new keeper. But euthanizing my betta was the hardest thing I ever had to do. I was in tears and heartbroken. But I had to do what was best for him.
On a less serious note. Definitely money, availability and willpower. Money, because the hobby has proven to be very expensive. I'm discovering new things I need to buy for my setup. Availability, because I only have a petsmart in my town that is either out of stock for things I need or just doesn't hold products in general. Willpower... because I want to own every catfish species I see posted on online stores.