Someone once told me that whatever you turn to in your time of need - depression, stressful times, disappointments, boredom - THAT is your God.
Leaning of food, sex, shopping, Internet usage, social media, etc.
I know a number of people who deal with their emptiness by trying to fill the void in their lives by various means.
One reads books every single solitary free moment of their day. Wide variety of genres. Page after page, hour after hour. Before work. Breaks. From the moment they get home from work until they go to bed. Always has 4-5 books in her backpack.
Another used to stop on the way home and rent 3-4 DVDs to watch until he fell asleep. This was done every single solitary day. Netflix has since given him a much easier fix.
One eats. All day. Breakfast. Lunch. Dinner - all full meals. They also eat regularly throughout the day and right up until they go to bed.
Yet another does online video games. Individually and on teams. Late into the night and all-day over the weekend. At work he incessantly blogs about it.
The last one is obsessed with social media: Facebook, Twitter, blogging, etc.
They don't know it (or didn't know it) but they are simply trying to fill a void in their lives. An emptiness that only Christ can fill.
Even Christians can have/exhibit this behavior. This is where the "I am a selfish God" part comes in.
God doesn't want to play second fiddle to something else that you willfully spend more time with than Him.
That is also true with sex. Premarital sex is wrong in and of itself.
But if you are married and your sexual activities go beyond acceptable to other areas (swinging, pornography, same-sex, etc.) that is also something God frowns upon and that activity drives a wedge between you and Him.
Also, there is Sexual Addiction. That can happen even within a monogamous relationship.
Basically, what it comes down to is this:
If your primary focus in time of need/emptiness/lonliness is something other than God, you have a problem.
If your precious free time is focused on something that occupies much more time than you give God per day, that, too, is a problem.
In a book called, "Did You Think To Pray," the author Kendall says, "Children spell love: T-I-M-E. What if God spells love: T-I-M-E?"