Because Chinese doesn't have voiced consonants. In Chinese, voiced /b/d/g/ are just variants of their voiceless counterparts. So you can't hear the difference between voiced sounds and voiceless sounds.
It's hard to explain and learn by text. Instead, I recommend you practice it by listening and imitating.
The site 首都大学東京 mic-J 日本語教育 AV リソース may be helpful. (AV stands for Audio Visual I think.)
This tutorial is specially designed for Chinese speaker who can't hear voiced and voiceless sounds. This is the old version. (5 years old, I think.)
This tutorial contains video explanations and 100+ exercises. Try and see how many you can hear correctly.
For Korean speakers who have the same problems, there is a Korean version, too.
If you don't speak Chinese or Korean, you probably need to learn Chinese or Korean first to be able to use it. )-: Anyway, try out Google Translator.
TIPS: The voiced consonant at the beginning of a sentence is not always fully voiced. There is a big chance it's devoiced. Instead, the voiceless sound at the same position becomes aspirated. In this case, Chinese speakers often have no problems hearing it.