I Want to Have Sex with my SO...

but he keeps saying he's tired. Always tired. Sigh. The last 3 times I've approached him for sex, he's said he's tired. I can't help it...after the third turn down I began to wonder if it was me. I asked if he was sexually attracted to me or if I'd done something to upset him....he said he loves my body and nothing is wrong, that he is just tired. 
Do you think that's all it is? He's just tired? I'm a bit sad and frustrated that I can't seem to turn him on.