Despairing About Meditative Regularity

I have set the amount of time I meditate (focused attention/shamatha) at merely 15 minutes per day. Despite this, despite all my best efforts, I seem to still skip a day or two now and then. **Why is this? Would changing the type of meditation help?** I had read positive emotions help develop a habit. When I meditate mindfully on the breath, there are no such emotions; doing compassion meditation, positive affect arises. **Would this affective benefit motivate a meditative practice?**