Karma..answer me this one too ..?

i asked question ....abt why some people are born handicapped or with some defect.. most answer were because of there earlier birth deeds.. if so.. i dont know what happend in my earlier lifes.. i dont know what i wil be in my next life.. so why i have to suffer(for those above) for something i dont know.. or probabaly i will never know.. why i should do good karma because i will get fruits in next birth... Even we human when punish, someone we tell them for which wrong deed we are punishing them..