Long term partnerships often go through phases where one or both partners are not sure they have made the right choice, but it can signal the end if there is suddenly a lack of enthusiasm for anything they have often done together. If one person begins to withdraw from common activities, is could be a sign they are no longer interested in spending time with their partner. If both of them are suddenly reluctant to do things together, it could be a mutual decision that the relationship has outlived its usefulness.
People change as they age and mature, and some relationships can be pressed too far to be able to weather this type of change. The couple could find their needs are going in a different direction, or just one person might decide they want something else. Mutual decisions are important when two people plan to live together for the rest of their lives, so a lack of interest on the part of one of them means they might no longer be planning to stay.
There are times when both partners lose interest in something, and it could simply be a phase of their relationship as a couple. It does not always signify they will split up, and it could be a step in the right direction. If they used to be excited to participate in extreme sports but continue to make excuses to avoid it, both of them might have reached the conclusion they need a new hobby.
There are many times when a person finds what they want out of life has changed, but those in a partnership might not want to upset the other person. It might be best if both of them were honest about their new feelings, and they could then have a better chance to work out a solution.