"I met this guy about three months ago. Everything was great for the first two months or so. We would text often and even talk on the phone a few nights a week . But now it seems that I'm always the one to initiate contact with him. Sometimes, he won't even respond until the next day. Why do you think that is?"
Ah, looks like you've become the "initiator" in the relationship - the person who is almost always the first one to call, the first one to text. Unless something tragic has happened in this guy's life or you're some crazy, obsessed person texting and calling him every 10 minutes, the reason you've become the initiator is because the feelings are no longer mutual and you've pretty much become just an option to him. You've been relegated to the position of, "Whenever I have time."
When someone is interested in you, if they're not calling or seeing you often, they will at least text you sporadically – once every few hours throughout the day – even if they're not a fan of texting.
A good indicator that they're interested is when you're getting simple random texts from them, not just texts when there's something to talk about. From "How was your day today?" to "Weather is really nice today." Or even texts that contain only an emoji. You know what I'm talking about. These texts are not great simply because of their content, they are great because they communicate, "I'm thinking about you."
If the guy you're seeing were really interested in you, he would be curious about your life and what you are doing. Not reaching out to you is his way of saying, “I don’t really care what you are doing. You are not important to me.” You could suddenly die or get kidnapped and he probably wouldn’t care what happened to you until your picture showed up on the news. Stop dating this guy.
Remember, the less he texts and calls you when you know he's free and able to, the higher the chances are that he's texting and calling other girls.