The man was created to worship Allah alone. A believer must be serious in his life and keep himself busy in things that will benefit him. Allah says in glorious Quran:
“And I (Allaah) created not the jinn and mankind except that they should worship Me (Alone)”[al-Dhaariyaat 51:26]
A believer must not engage in matters that may harm him and distract him from doing the acts of worship and from that for which he was created. He must take his world and hereafter in consideration before doing any stuff.
The basic guideline concerning permissible things according to the scholars is that whatever distracts one from obligatory duties or becomes a means of committing forbidden actions; thereby becomes haraam.
As for the things that distract one from doing things that are obligatory but are not means of propagating haram, these are considered to be makhroo.
Whatever does not distract from either obligatory duties or mustahabb actions is permissible in principle.
Supporting a sports team makes a person forget about Allah and his obligatory duties apart from wasting his time and energy. For example, a person may forget and neglect his obligatory duties like a prayer by spending time in watching them play. The give high priority to their team rather than the worship which is unacceptable in Islam.
If a matter of supporting reaches this level then without any doubt it becomes haraam. In addition, this support is accompanied by an attachment of the heart, and love and hatred based on that.
And Allah knows best.