When you check on your phone and you see a new message/email/tweet or whatever, or better yet it checks on you with a short vibration in your pocket, note the feeling. I think most of that can be isolated to a dopamine spike; assuming it’s a message that you wanted to get, your brain is rewarding itself. It’s probably fair to compare this to the sensation people used to get when receiving a call from a friend (before caller id), or before that a letter. Due to the lower frequency, and probably greater content, of those communication forms I wonder how much more the pleasure was amplified. Nevertheless, the frequency that many of us experience it now is much higher. And the impact is large enough that many develop an involuntary urge to check their device; this is the addiction mechanism at work obviously.
I don’t know if it’s fair to classify this as truly an addiction or whether all addictions can be considered Bad. I do conjecture that the described state of mind is likely not a desirable one, when you step back and consider it objectively. The benefits one receives from having the device handy are likely overshadowed by the side effects on the rest of their life, which may not be obvious until cutting out the behavior. Not everyone is equally susceptible to the effect but for those that are particularly sensitive, for whatever reason (I am included I believe), they should consider cutting out such devices from their lives.