To vote?

Being a Portuguese citizen living in Germany I have the right to vote for the Presidential and Parliamentary elections in Portugal. In Germany I have to restrict my participation to the Communal elections. The question thats nags my head is the following: Why am I allowed to vote for candidates in Portugal whose policies will hardly affect me? Paradoxically, why can't I vote for the highest representative of the country where I currently live and whose decisions (not all but quite a lot) affect me directly? I have therefore doubts about the fairness of current electoral systems for foreigns of both Portugal and Germany (and probably in other European countries).

One option could be to drop the participation in Portuguese elections and be able to vote for the general German elections instead. On the other hand I will be always a Portuguese no matter where I live. This means that I have preferences on how my country is represented in a context of International cooperation or its policies regarding Portuguese emigrants. Similarly, being Portuguese, some German laws do not apply, so why should I have a word to say.

I think the way to solve this is to answer the question: When does my ability to decide on the future of Portugal stops and when does my political responsibility in Germany begin?

PS: Today, the Portuguese of the world decided on their new president.