It can include:
A change in a relationship with another person
Moving to somewhere new
Realizing your expectations about your job or your life aren't the same as the reality
Not being able to accomplish something we thought that we should achieve or be able to get (no matter how impossible the expectation)
This isn't an exhaustive list, but it's something I see frequently with clients.
The relationship change can be especially powerful. For example, a break-up is often a very painful event even when it's something the client wanted. In some cases they miss their partner or what they thought they had with them (before everything fell apart), and in others they may be traumatized by the relationship and grieving the loss of time and sanity from it. In some of the worst cases, a friend's personality may change drastically (which the Covid phenomena seems to really be a catalyst for many cases!); in this scenario, the friend becomes a completely new person. Imagine: Someone who was your rock and confidante suddenly becomes a shadow of their former selves, whom you loved. This new person may seem more shallow, manipulative, selfish, or any other adjective which elicits disgust. You've lost them, but they're still right there. You may ask yourself what you could do or should have done to "save" them, but the answer is that they've made their own choices. It's like they've died, but some highly unpleasant doppelganger has taken their place. Even worse, you may never have seen it coming.
Realizing that your expectations aren't the same as reality can also be a terrible loss. You may think that a person or situation is fine or even ideal, and then come to understand that it's not what you thought it was. There's a grief in learning that things are not only completely different, but even not the thing that you'd hoped it would be.
Coming to terms with things about ourselves can even cause grief as we let go of old parts of ourselves and adopt new pieces of our inner puzzle. We might feel like we've lost something valuable, such as a carefree spirit or the ability to socialize without anxiety. In reality, we've learned new truths which show us a deeper, more complex version of how the world truly is. This can make us fuller, psychologically richer people when we find the meaning in these changes and what they mean for us.