It's just something that happens in life. People will always leave. Maybe your relationship with them isn't as good as it was before. You can definitely try to make them stay, however, you can't ensure it every time. It's natural that people will leave and it's okay to be sad about it or something, but don't try to think about it for long. It may seem difficult at the start but it'll be alright. I know this answer may sound very generic but it is what it is.
Emotional connection is often what sustains a long-lasting relationship and if this is no longer there, this may be another reason why people chose too leave the person they love.
Answers & Comments
It's just something that happens in life. People will always leave. Maybe your relationship with them isn't as good as it was before. You can definitely try to make them stay, however, you can't ensure it every time. It's natural that people will leave and it's okay to be sad about it or something, but don't try to think about it for long. It may seem difficult at the start but it'll be alright.
I know this answer may sound very generic but it is what it is.
Answer:
They no longer feel an emotional connection.
Emotional connection is often what sustains a long-lasting relationship and if this is no longer there, this may be another reason why people chose too leave the person they love.