Short answer, because it's supposed to.
One of my favourite quotes that always sinks in immediately after I read it is "If it were easy, everyone would do it." It's so true, nothing worth working for will ever come without the work. I get dragged down by the daily grind all the time, it's a constant struggle. Somedays the results show and make it all worthwhile, somedays you question why you're doing it.
The negativity can get to you, but it's important to keep your goal in mind and keep moving forward. Everyday will bring a new challenge, new obstacles to maneuver around and constant monotonous tasks. These are the things that can slowly eat away at your motivation and enthusiasm but are simply vital for your business to succeed. To better deal with the daily grind, you have to fall in love with the process. Take each new challenge a a reward for tackling the last one. Treat every obstacle as an opportunity to prove you're great at what you do. View the monotonous tasks as lessons to sharpen your skills, becoming better than the competition with each completed duty.
The opportunity to live your dream life isn't given away, it's earned. This ultimate prize comes with a higher risks and greater mountains to climb. Hard work is the price of admission for the dream ride. You'll have to work your butt off day in and day out, sometimes it will be hard. Frankly, most days it will be hard, but that's the point. If you want what most people don't have, you have to do what most people won't do. So keep working towards your dream, as soon as you make it, it will all be worth it.