Because you’re relying on willpower.
What
 I’ve come to realize after working with hundreds of people that were 
lacking self-discipline is that they did not have any reliable structure
 in their days that made it easy to take action, but rather were relying
 on willpower.
Willpower is somewhat like a 
muscle, a source of energy that depletes over time. Relying only on 
willpower could serve in the very beginning when motivation is still 
very high. Whatever goal you have set for yourself, you are excited to 
give it a shot and eagerly do the first steps.
Over time, however, your willpower will decrease.
 This depletion can be seen over the course of several days and weeks, 
but also in a single day, as your willpower starts off high in the 
morning and declines towards the evening.
At
 this point it will be very difficult to pack your bag for the gym, to 
resist that delicious pizza around the corner, or to finish that book 
instead of wasting time on social media.
You then wonder why you don’t have any self-discipline.
So both motivation and willpower are actually rather low ranking traits on the scale of consistency and productivity.
Let’s look at the four levels:
- Willpower/Motivation: As described, having good levels of willpower can be useful to make yourself do something and overcome laziness. However, just like motivation, willpower is unreliable, so this is actually the lowest level of productivity and consistency.
- Discipline: Discipline implies that you have to bring up a certain degree of willpower to overcome laziness. It is better than sheer willpower alone, because you regularly bring up that willpower and get used to doing it, especially if you don’t feel like it. However, it’s not reliable either, since on some days you manage to discipline yourself, on others you don’t.
- Habits: Habits are the cornerstone of consistency. You shift from conscious effort and willpower to implementing habits that take over and run on autopilot. The key is to implement habits by designing your environment and it will be easier to actually stick to them and make them a part of your day.
- Identity: This is the ultimate level of consistency. It is an evolution of habits. Once you are consistent with your habit for a long period of time, these habits will turn into your identity. It will not be a habit for you to e.g. go to the gym, you simply will go to the gym no matter what because you are a person who goes to the gym.
At the last stage, there will be no questioning, no laziness, you literally just do it. You would have to use willpower to not do it.
This is where you want to be.
 Whatever goal you have. Don’t think of the result, but rather the kind 
of person you have to become and the tiny steps that are needed to get 
there.
Reading some pages, doing some minutes of exercise, cooking a healthy meal yourself… taking small steps in the right direction on a daily basis establishes lasting habits that will ultimately shape your identity.
What is the first step to get there?
Choose a keystone habits and make it stick.
A
 keystone habit is a habit that once established can have a ripple 
effect, whose benefits expand to other areas of your life and thus over 
time improve it as a whole.
Charles Duhigg describes them in his book “The Power of Habit” as, "small
 changes or habits that people introduce into their routines that 
unintentionally carry over into other aspects of their lives."
What are examples of keystone habits?
One example is exercise.
 Regular work-outs have several side effects. You develop a better 
posture and feel better, which is easily noticed by your surroundings 
and can help you in your dating life. It also makes you take care of 
your nutrition and how you treat your body. It increases self-discipline
 and thus affects your progress on difficult tasks and so on.
You will then notice how it will influence other areas of your life and allow you to progress in these areas.
How do I actually stick to my habits?
By making it easy to stick to them through intelligent design of your environment.
A
 good example of how to use Environmental Design is your phone. The 
first thing many people do after waking up is to reach for their 
smartphone and check messages, e-mails or notifications from social 
media. This sets you in a reactive state and will prevent you from 
focusing on your true priorities.
Therefore, before going to bed, intelligently design your environment.
Put
 your phone in airplane mode and place it somewhere impossible to reach 
from your bed. Prepare your to-do list with the 5 most important things 
you want to achieve the following day and place it somewhere you can see
 it as soon as you wake up.
After waking up,
 you can directly focus on the tasks that lie ahead. Should you still 
instinctively reach for your phone, there will be no distractions 
available due to the airplane mode.
This 
will allow you to start your days in a productive instead of reactive 
state. This is one of the most important aspects of productivity, 
because remember:
“If you commit to nothing, you’ll be distracted by everything”
To
 summarize, discipline and willpower are not reliable. Focus on 
implementing daily habits which make taking action inevitable and 
automatic.
Hope that helps,