1. When you are doing it everyday and that is the first thing you want to do when you get home for no apparent reason other than extrinsic pleasure.
This of course include binge eating, gaming, playing the guitar et cetera. These are activities that one can engage in and they are all good activities to relieve stress with. However, it should not affect your sleep, it should not affect your work, and your relationship with your friends as well as family. Many a times, activities such as playing the guitar, gives us a recluse that we often dismiss we have an addiction. While needing a break is true, and there is nothing wrong with wanting to go home just to game again, but if these activities take a higher priority than what is (world view rat race styled) morally correct, then it may be an addiction.
Why so? Because we are human beings and we live in such a world. If we do not put the right priorities up ahead, we will not be able to survive the onslaught from work and the stress that ever so piles up.
2. When you talk about it all the time.
Now, there is nothing wrong with talking about, let's say, soccer during lunch break. Nothing wrong with chatting around your boyfriend and his every so sweet and kind actions towards you. I mean, if someone is sweet to you and does such awesome things, there is nothing wrong with it. Especially when you're in love.
However, it crawls and evolves into a problem if what you are talking about is constantly an activity even during work. Or you talk about it all the time even when the topic is on something else, or excludes people within the social circle from the conversation.
3. If you are thinking about it right now while reading this post.
If you even need to suspect whether it is an addiction, it probably is.
Because if it isn't why would such a thought even appear in your mind? It just won't happen.
Loving someone and being head over heels is not an addiction. It will be better if we classified it as infatuation, or being in love. It does not equate to an addiction. For example, just because your mother talks about you to her friends at work doesn't mean she is addicted, it could be that you really have a lot of problems that is alarming her, or that she is really proud of you. Or something.
Humans are not an addiction. You can only be addicted to an activity. We have a wide diverse number of words, choose the right one to describe an "addiction" towards humans.