Endless business requirements, desired features, market pressures… There is always more work that needs done. Sometimes, it may feel like endless sprints where you thought you saw the finish line, but every time you round the track they tell you there’s another lap. You dig deep and pull together the energy to continue to sprint, but again, you round the last turn and push forward toward the end and people on the sidelines are yelling “another lap, another lap” and you check to see if you can find the energy to keep going, but you can’t. You keep thinking you’ll have a break but it never ends. The sprints are seemingly endless back-to-back cycles and you see no end in sight. You need to catch your breath, you need water, you’re getting blisters… You’ve burnt out. Finding a sustainable pace is important.

Source: https://www.scrumalliance.org/community/articles/2011/december/sustainable-pace-trusting-your-teams