Preparing sample for SDS PAGE
Usually you would want to keep the amount same, not the concentration. However, if you still want the concentration to be the same then you can add suitable amounts of PBS or your lysis buffer. 

For e.g. if you have two samples with 1mg/ml and 4mg/ml concentrations and you want to load 20μg of total protein, then you can take 20μl of first sample and add 5μl of 5x loading buffer. For the second sample, you would take 5μl of the sample, 5μl of 5x loading buffer and 15μl PBS.


  Why 1x?


Because researchers before us found this composition to be optimal (or just simply stuck to it as a matter of practice).


  Would it be a problem if the final concentration of the loading buffer is more than 1x? 


Apparently not, in my experience if it is up to 1.5x. Never checked higher concentrations.