For me it is the chore factor, the feeling that it's something bothersome that has to be done. One way around that I've found is to convert it to a desire - saying to myself "I deserve clean, healthy teeth" is enough motivation, for me, to pick up the floss. Similarly, "I deserve a clean, tidy home" gets me picking up the vacuum and mop, "I deserve a healthy body" is a useful mantra for eating well and exercising.