There are a few things that affect how you perceive your voice when you’re singing in the shower. If you’re taking a hot shower, then the air is much moister (and heavier) than normal, so your voice is travelling through that air before it reaches your ears. Of course, you hear yourself inside your head too, which is why most people don’t like hearing themselves on a recording. But beyond all that, the acoustics in bathrooms are very distinct, and you voice reflects and echoes off those hard surfaces. Those reflections from all the surfaces add a certain characteristic to your voice. That’s likely why you prefer hearing yourself sing in the shower.