When O Sensei said that the main purpose of aikido practice is not to be strong,i'm just thinking about my purpose in aikido.. Is it wrong if I practiced to become strong? Of course not strong 'outside' but strong 'inside' of me.. It's still become question inside of my head.. Thanks for answer my question..
No it is not wrong. Consider it a point to start.
See if there are changes in your thinking as you progress.