Have I committed a sin by treating a friend coldly because he had not welcomed me previously, and what should I do about it?

Among the good deeds that a person might overlook is meeting one's brethren with a cheerful face, as the Prophet (peace and blessings be upon him) said: "Do not belittle any good deed, even if it is meeting your brother with a cheerful face." A believer does not reciprocate evil with evil; rather, they forgive, overlook, and repel evil with what is better, as Allah Almighty says: "So whoever pardons and makes reconciliation - his reward is due from Allah," and His saying: "But whoever patiently endures and forgives - indeed, that is of the matters [requiring] determination." And the Prophet (peace and blessings be upon him) said: "Allah increases a servant in honor only if he forgives." Therefore, you should take the initiative to greet your friend with cheerfulness and welcome to remove any ill feelings.