Now, if the first point is that it is not your responsibility to change these people, the second one is that God is reading the heart and he sees something that you cannot see about them:
Again, one man thinks some days of more importance than others. Another man considers them all alike. Let every one be definite in his own convictions. If a man specially observes one particular day, he does so “to God.” The man who eats, eats “to God,” for he thanks God for the food. The man who fasts also does it “to God,” for he thanks God for the benefits of fasting. The truth is that we neither live nor die as self-contained units. (Romans 14:5-8 NIV)
That is what we are getting at. Paul is saying that God can read hearts and you cannot. These distinctions and differing points of view arise from honest convictions, which God sees, even though you cannot. The individual is not simply being difficult simply because he does not agree with you. He is acting on what he feels is right, so give him the benefit of the doubt on that. Believe that he is as intent on being faithful to his Lord and true to him, the same as you are. If he feels able to indulge in some of these things you think are not right, then at least see him as doing so because he truly feels that his Lord is not upset with him because of it. On the other hand, if he does feel limited and he feels he should not do certain things, leave him alone and not get upset with him because of his views. Remember that he really feels that God would be displeased if he did those things. He is being true to his convictions. The apostle makes clear here that every man should have that kind of a conviction, if he acts this way. “Let every man be fully persuaded in his own heart”
Do not just act from tradition, because you were brought up that way or because you just feel it is right. Find some reason in the Scripture for it. Look for support out of the Word of God. You may change your mind as your understanding of truth develops, but at least it will be grounded on the convictions of your heart and of the mind.
If interested, you can download the entire study of The Letter to Christians at Rome