Why do I wake up thinking about my ex?

You just miss the fellow. Given enough time, and levels of distractions, your mind will ease off his/her name eventually. This was the case with me, and I never figured I'd stop thinking of my ex.

It is because you spent some time with that person and you had moments together. Time heals wounds. You can also find yourself a hobby and focus on that maybe it would help.

I believe there's a theory that our brains tend to get stuck on 'unfinished business', which is why we routinely get songs stuck in our head, and the suggested solution is to listen to them to 'complete' the process. But of course, people and relationships are not songs, so we can't go back and complete them, so the 'what if' scenario is always going to come back to you. There's also a school of thought that dreams are what help us with memory processing (people who've slept can help recall memories better than those who haven't), so sleeping is going to help you remember and process events in your lives. Additionally, every morning, after having slept and forgotten about reality, reality comes flooding back to you, including all those memories. It's like when you wake up the morning, think everything's OK for a bit, and then suddenly remember that exam you have coming up. But there is no exam, just that small reminder of that thing in your life that's a reason for you to feel that little bit not-OK. And as a species we're obsessed with finding that happiness, so we will always focus on the not-OK things as a reason to motivate ourselves to make things better. I mean, it's not a fun way of going about that, but it's a way.

It is most likely due to the fact that you've grown an attachment to them and it feels like you won't be able to get that attached again to another person.

You wake up thinking about your ex because of the positive impact he has had in your life. Each positive interaction while the relationship was intact enforced more caring and positive feelings regarding this person, making them an important part of your life. When they are gone you realize they became an important source of happiness throughout the time you spent together. Fortunately this effect can be achieved with any person you befreind or fall in love with.

It may be because of past experiences. For example when a relationship of mine ended I kept thinking of him when I woke up because he would always leave me good morning messages. Think if there was every something you two did or he did for you in the mornings, even if you texted each other goodnight.

Because you haven't let go yet. This can take some time, and that's how it is. It's hard, but it will go away.

Because, for a large amount of your life, they were there. They took up a lot of your thinking and feeling and now, you're just adjusting to the change. It's normal, and it's by no means obsessive. Don't worry.

Getting over an ex is tough. They had a large role in your life and it's hard when that role is empty. For a long time, you are going to miss them, random seconds of everyday. But eventually there will something else, someone else to think about.

Because this person has a deep impact on you or your life. Either you are hurt and in a lot of pain by them or they were once a big part of your life and now are no longer.