Why did he keep staring at me?

Last night, I was taking the tram home, and this extremely young guy (about 20) was also there at the tram stop. I walked past him, but not too far ahead and then just faced the tram tracks, waiting for the tram to arrive. But through my side vision, I noticed he seemed to be glancing at me, and every time I looked in his direction, he'd look away.

Then the tram came and he sat a few rows behind me, was an empty tram. I listened to music on my iPhone, trying to wonder why he was staring at me… Then finally when my stop arrived, I got up, walked to the back door of the tram, and as I was waiting for the tram to stop, I noticed through my side vision again he was actually turning around, looking in my direction. I couldn't help it anymore, I just looked back at him, and he WAS looking at me, straight in the eye! He had a slight smile. When i got off, I was just thinking wtf did he keep looking at me…

some background info is that i'm 17, and girls often describe me as cute.

Added (1). yeah, i'm a guy. :/

If you're a man, he may be gay for you. If you're a woman, he may be hetero for you.

When men look at women/men like that, it's usually because they find them attractive.

What else would you imagine?