The commemoration of the Ahlulbait (Peace be upon them) is amongst the best deeds that the religious texts have emphasized on, and it is narrated that there are great rewards for those who perform it.

What the lovers of the Ahlulbait (peace be upon them) do in the acts of mourning is expressing their grief and pain for what happened to Imam Hussein and his household on the day of Ashura. It is not something strange. It is, rather, an expression of the love and loyalty that Almighty Allah has ordered the Islamic nation to hold when He said: "Say (O , unto mankind): I ask of you no recompense for this, except love for the kinsfolk." (23:42). Sunnis and even non-Muslims participate in these commemorations since the loyalty to the Ahlulbait and loving them is not limited to the Shia.
These commemorations are not a tradition but it is an implementation of the orders of the Imams of the Ahlulbait to their followers, when they ordered them to hold such commemorations. It is narrated that Imam Ja'afar al-Sadiq (peace be upon him) said: "Enliven our commemoration. May Allah have mercy on those who enliven our commemorations". There are many other such narrations.
Yes, people and nations differ in their expressions of sadness, since the aim is to commemorate these tragic incidents, as means of sharing their grief and sorrow.

Shias mourn all the martyrs of Karbala. They remember their faithful dedication to sacrifice themselves with praise and respect, whether they were among the Ahlulbait or among the other companions. It is customary in some Shia communities to specify a day within the first ten days of Muharam to remember one of the martyrs of the companions of the Imam as well as a specific day for Abbas bin Ali (peace be upon him) and other members of the Ahlulbait.
The Ziyarats that were narrated from the Ahlulbait (peace be upon them), were specified for Imam Hussein (peace be upon him), while they also mention the companions with praise and Salam. It is natural that Shias will mainly mourn for Imam Hussein (peace be upon him) since he was the Imam and the rest were his followers and companions, in addition to his relation to the Messenger of Allah (peace be upon him and his holy progeny).

It is reported in religious texts not to wear black clothes, which suggests that it is disliked to do so. However, there is no doubt that it is preferable to wear black clothes to mourn what happened to Imam Hussain (peace be upon him) and his household and companions as this represents loyalty and love for the Ahlulbait (peace be upon them).

It is permissible to seek the blessings from whatever is related to the Prophet and the Imams (peace be upon them all). Seeking one's wishes from them is basically seeking intercession from them because of their high rank and closeness to Almighty Allah.

There is no doubt that we should commemorate the tragedy of Ashura and express our grief and sorrow over what happened to Imam Hussein (peace be upon him) and his family and companions; and the Imams (peace be upon them) emphasized this in a well-known manner that is recorded in the books of narrations.
However, differences have occurred in respect to some practices and rituals, and opinions have differed about them for some time.
Our advice to all the believers is that they should refer in this issue to those Maraji who they follow and fulfill the conditions of Taqleed, and they should distance themselves from conflicting and quarrelling over it, as a believer may commit a sin as a result of such arguing. Rather, it is best at times to leave such quarrelling when there is no benefit in it. This may even cause harm and lead to animosity and hostility between believers.

It is recommended to perform the acts of Arba'een on the 20th of Safar.