there are a lot of reasons, but the main reason may be that he is playing you. he wants you to suffer while you wait for his reply. i know how you feel, its so annoying and childish. when he texts you, you text right away right. how rude of him. unfortunately, there is nothing you can do. except this..
stop looking at your phone!!!! i bet you look at your phone every second that passes by waiting and wondering perhaps if your phone is working or if the battery might be dead.
another favorite excuse to look at your phone it to look at your sent folder to make sure that you really sent him the last text message. and you check, and sure enough, there it is, it was sent. except that he is not replying back.. why is he not replying back, is he talking to some other girl maybe?
hahahah, dont play mind games with yourself. just put the phone away and continue with your life. keep your mind busy to stop thinking about the cellphone. put it down for a while... and when you come back, you'll see, his text will be there. later you can ask him why it took him a long time to text you back. he might have a good excuse
tell me, why do you want him to text you so bad?