Because that's when growth happens. If you're not interested in growth then you should just stop here.
Growth happens gradually through consistent small incremental improvements. If you are not feeling the friction and the resistance in movement, it's a sign that you are not growing. If you are not feeling the resistance, then you might be standing still while your competition moves forward, providing more value and thus eroding the relative value you provide.
When asked about how many sit-ups he does, Muhammad Ali had this to say:
"I don't count the sit-ups. I only start counting when it starts hurting because they're the only ones that count. That's what makes you a champion."
Imagine if you developed this kind of mindset.
By default, you want to save energy. Saving energy translates into laziness. Saving energy comes from the mindset of survival. It assumes you have a limited amount of energy and to survive, we must save some rations for tomorrow. It's not wrong. It's a healthy mindset to create resilience so you can survive to fight another day. But that is not the mindset required when you want to grow and thrive. Imagine if you trained yourself to do the uncomfortable things. Sooner or later, what seemed uncomfortable to you will begin to feel easy and natural. Imagine what you could achieve if you carried this mindset to everything that you do.
Normally, when you feel that tinge of uneasiness, when you're not feeling motivated to do that thing that needs to be done, the default action pathway is for you to distract yourself to so you can avoid the anticipated pain of doing the work. The more you avoid it, the more mental stress it creates for you. Instead of avoidance, you need to embrace the difficulty. It is what separates the wannabes for the be's. The people who ARE.
It's easy to do anything when you are feeling inspired, motivated, and energetic. Nobody in the world would have a problem doing the work when they are in that state. What really counts and gives you an edge over your competition is what you do when you don't "feel" like doing the work. Ultimately, the real competition is between you and your previous self. That is where growth is happening.
If you want to grow muscle, you gotta train against resistance. If you want to be healthy, you want to train your body by withholding food from time to time. All these growth experiences are painful in the moment. No pain no gain.
So every time you hear your mind say "Nah, I don't feel like doing it", use that as a trigger to do the opposite. The more you train this way, the more comfortable you'll get doing what is currently uncomfortable. The more you do that, the more skilled you'll become. The more skilled you become, the more action you'll take. The more action you'll take, the greater the probability that you'll create value for others. The more value you create for others, the more value you'll get.