A timely reminder for anyone who has ever possessed any of the characteristics of Ḥizbiyyah (sectarianism).
It is not upon anyone to ascribe himself to a particular Shaykh and to have wʿalá‘ with those who follow him, or to have ‘adaa based upon that. Rather, he should have wʿalá for everyone from the people of īmān (i.e. he is a Muslim) and is known to have taqwá (piety and obedience to Allāh and that which He has ordered and prohibited), from the Shaykhs, or from others.
THE BASIS OF RIGHT GUIDANCE
Shaykh al-Islām Ibn Taymīyyah (d.728H) - raḥimahullāh - said:
"The Religion of the Muslims is built upon following the Book of Allāh, the Sunnah of His Prophet (ṣallallāhu ʿalayhi wa-sallam) and that which the Ummah has agreed upon. So these are the three infallible uṣūl (fundamentals). So whatever the Ummah differs in, then it is referred back to Allāh and His Messenger. Thus, it is not for anyone to set up a person for the Ummah, and to call to his way and form wʿalá’ (love, loyalty and allegiance) and ‘adaa (enmity and hatred) based upon that, except for the Prophet (ṣallallāhu ʿalayhi wa-sallam). Nor is any speech set up for them based upon which they form wʿalá’ and ‘adaa except for the Speech of Allāh, and that of His Messenger, and that which the Ummah has agreed upon. Rather, this is that practice of the people of innovation, who set up a person or a saying, with which they cause splits in the Ummah; forming wʿalá‘ and ‘adaa based upon that saying or ascription." 
Imām Ibn Taymīyyah - raḥimahullāh - said:
"It is not upon anyone to ascribe himself to a particular Shaykh and to have wʿalá‘ with those who follow him, or to have ‘adaa based upon that. Rather, he should have wʿalá‘ for everyone from the people of īmān (i.e. he is a Muslim) and is known to have taqwá (piety and obedience to Allāh and that which He has ordered and prohibited), from the Shaykhs, or from others. No one should be particularised with an increase in wʿalá’, except if he sees in him greater īmān and taqwá. So precedence is given to those whom Allāh and His Messenger have given them to. Allāh the Exalted said:
"O Mankind! Indeed we created you from a man and a woman and made you into tribes and nations so that you may know one another. Indeed the most noble of you with Allāh are those that have the most taqwá (piety)" [Sūrah al-Ḥujurāt 49:13] 
SECTARIANISM AND MISPLACED LOYALTIES
Shaykh al-Islām Ibn Taymīyyah said:
"Indeed the people of the Sunnah do not follow anyone (unconditionally) other than the Messenger of Allāh (ṣallallāhu ʿalayhi wa-sallam); who does not speak from desires, rather it is nothing less than Revelation sent down to him. So it is obligatory to affirm all that he informs, and to obey all that he commands. This status is not given to anyone else from this Ummah. Rather, everyone's statement can be taken or left except for that of the Messenger of Allāh (ṣallallāhu ʿalayhi wa-sallam). So whoever sets up an individual other than the Messenger of Allāh (ṣallallāhu ʿalayhi wa-sallam); such that the one who loves and agrees with him is reckoned to be from Ahl al-Sunnah wal-Jamaa‘ah, and the one who differs with him is reckoned to be from the people of innovations and sectarianism - as happens with those groups who follow certain Imāms of kalām (rhetorical speech) - then the person doing so is from the people of innovation, deviation and sectarianism." 
He - raḥimahullāh - further stated:
"Indeed many - from the people who say such things - follow their desires in this, in order to elevate their egos, or to gain leadership, or whatever is connected to it. Their intention is not to make the Word of Allāh uppermost, nor is it to make the Religion purely for Allāh. Indeed they have hatred for those who oppose them, even if it is a mujtahid (a scholar striving his utmost to arrive at the truth) who is excused and whom Allāh is not angry with. Likewise, they are pleased with those who agree with them - even if it is an ignorant person with evil intentions - who neither has any knowledge or good intent. This leads them to praise those who Allāh and His Messenger have not praise and to censure those whom Allāh and His Messenger have not censured. It also causes them to have wʿalá’ (love and allegiance) and ‘adaa (enmity and hatred) based upon their own self-desires and not upon the Religion of Allāh and His Messenger ... So whoever does this will only bring about fitan (trials and tribulations) between the people." 
PRESERVING LOVE AND UNITY
Shaykh ʿAbd al-ʿAzīz Ibn Bāz (d.1420H) - raḥimahullāh - said:
"The Religion of Allāh orders to judge (with justice) in all matters and not to have wʿalá’ (love, loyalty and allegiance) with your brother merely because he agrees with you, or to have ‘adaa (enmity and hatred) merely because he disagrees with you on a certain opinion or issue. This is not from justice at all! The Companions (raḍī Allāhu ʿʿʿanhum) differed in many issues, yet this did not affect the overall happiness and sincerity between them, nor their wʿalá’ and love for each other - may Allāh be pleased with them all. So the Believer acts upon the Sharee‘ah (prescribed and divine Islamic Law) of Allāh, follows the truth, gives precedence to the evidences before anyone - yet in doing so, he does not oppress his brother, nor negate doing justice to him when he differs regarding any issue of ijtihād (personal reasoning) in which the evidences are not so apparent. Likewise is the case for those issues in which differences occur due to different interpretations of the text. So in such instances, he is excused and it is upon you to sincerely advise him and to love only the good for him and not to split-up, nor to have enmity or hatred between you and your brother - and there is no might, nor power except with Allāh." 
 Majmoo‘ul Fatāwá (20/164) of Ibn Taymīyyah.
 Majmoo‘ul Fatāwá (11/512).
 Majmoo‘ul Fatāwá (3/346-347).
 Minḥājus-Sunnatin Nabawiyyah (5/225).
 Majmoo‘ul Fatāwá wal-Maqālātul Mutanwwi’ah (1/346).