Have you been doing the same exercise programme for as long as you can remember? The majority of people stick to the same programme that they are familiar with because they are in their comfort zone. One of the biggest ways to overcome training stagnation is to break that barrier and step out of the comfort zone.
There are multiple ways to keep exercise invigorating both for the mind and body. You need to challenge the body and keep it guessing otherwise it will become more and more efficient at doing what you are doing. That might sound silly but in order to progress you need to step out of your comfort zone by applying progressive principles. Otherwise you are simply training for maintenance at best.
Depending on the level of training experience I would normally recommend changing exercise programmes every 3 weeks. Within a 3-week cycle I like to rotate different variables to stimulate additional response from the programme.