Why I run every day
Consistency is one of the pillars of endurance training. Stacking succesful week after succesful week. In order to be consistent, you need to have discipline to get out the door and follow your training plan. It may sound simple, but there are those mornings where you really need to convince yourself to put on your gear... and venture out into the rain. Or where you just feel sluggish, sleepy and just want to stay in bed.
The cure
One possible cure I found against this is to start a running streak. Counting the number of consecutive days (or weeks) where you reached your goal. As soon as you get that ball rolling, it's harder to stop. At least, that is what I noticed for myself. Yes, it can still be hard to get out of bed and go for a run. But now there is not only a voice telling you that taking a day off wouldn't be the worst, there is also a different voice telling you that it would mean your streak is going to end. As silly as it may sound, I find it does work.
“As soon as you get that ball rolling, it's harder to stop.”
The training plans I have followed in the past, all from McMillan Running, have each had 5-7 days of running in them per week. There are five days of running and two days of either complete rest, cross training or easy running. When I was testing with six days of running (and having a set day of rest, for example Friday), I found myself shifting that day around based on how I felt. Sometimes I would switch a harder workout with a rest day "because I wasn't feeling it". Allowing yourself to get out of hard workouts like that certainly doesn't build any mental strength either.
On top of that, since I was taking complete days off anyway, it was easier to grant myself a couple more every now and then. Which felt like a logical and okay thing to do in the moment, but I would regret it the rest of the day. Why? Because I knew I made the wrong call and I wasn't training at the top of my abilities.
Daily versus weekly
I can run seven days a week. Not everyone might be able to (considering scheduling issues, for one). So for me, having a streak based on running every day works.
Mind you, a day of running could be as simple as going out for a ten minute run. If that is what you can handle and suffices, go for it. Maybe for you counting the number of weeks in a row where you reached your weekly goal works, such as running three times a week. Or maybe you have more discipline and you don't need a streak at all. Good on you, I'm jealous.
As an aside, I don't think a monthly running streak will work. One part of a running streak is the satisfication you get as the number goes up. When counting months, you don't get that satisfication. It takes too long. Additionally, if you want to run, say, fifteen days every month, it can still be hard to get yourself moving at the beginning of the month. Which would leave you with no other option than to do the bulk of running at the end of the month. That can still happen in a week, but the feedback loop is shorter. Each week you can learn from the previous one.
Another thing I notice in the morning is the decision overload of which route to run. I try and take some time on Sunday to figure out which routes I'm going to run the next week, based on the training that's scheduled.
So, if you have a running goal that motivates you but you still have trouble getting out the door: try starting a running streak. Succes is not guaranteed, but it doesn't cost you anything either!