Proverbs 18:1 (ASV) He that separateth himself seeketh his own desire, And rageth against all sound wisdom.
The word separates in some translation like the NIV and NLT is translated as unfriendly person/people and the NKJV it is translated as Isolates.
In the Stong concordance it is defined as: 
H6504 פָּרַד pârad paw-rad’ A primitive root; to break through, that is, spread or separate (oneself): – disperse, divide, be out of joint, part, scatter (abroad), separate (self), sever self, stretch, sunder.
We can see from that, that being unfriendly is one of the reasons for separation/isolation. Isolation is a very good indicator of selfishness. It shows that the person is seeking their own desires, comfort, pleasure, etc.. When one is selfish one does not care for those around them and becomes unfriendly, rude and keeps to themself. Also on the other side, who wants anything to do with an unfriendly person? 
Either way selfishness results in a lifestyle of isolation and separation which is wrought with great danger and shows lack of wisdom.
When one isolates themself from people and the fellowship of believers, the following problems arise:
• One will not grow in the fruits of the Holy Spirit “Galatians 5:22-23 (KJV) But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith, 23 Meekness, temperance: against such there is no law.” and for example one remains selfish as the problem will not be exposed and there is no encouragement or exhortation to change. We all need to have other people to exercise the fruits like kindness, gentleness, love, meekness…
• One removes themself from what GOD desires. He desires us to fellowship not only with Him but also one another. 
• One exposes themself to become easier prey to the enemy of our soul the devil.
• One does not grow in and use the gifts GOD has given us to help each other.
• One removes themself from support of the family of GOD.
• One stops the blessing to flow, as one can not receive or give. For example receive and give: encouragement, acts of charity, being shown kindness…
• One misses out on what GOD is doing in the body of Christ.
• One misses out on the celebration of communion and worshipping the LORD together.
• Can you think of other things?
Colossians 3:15-16 (KJV) put some of it together “And let the peace of God rule in your hearts, to the which also ye are called in one body; and be ye thankful. 16 Let the word of Christ dwell in you richly in all wisdom; teaching and admonishing one another in psalms and hymns and spiritual songs, singing with grace in your hearts to the Lord.”
We can see that isolation is unwise.
Therefore let us do, as it says in Hebrews 10:23-25 (KJV) “Let us hold fast the profession of our faith without wavering; (for he is faithful that promised;) 24 And let us consider one another to provoke unto love and to good works: 25 Not forsaking the assembling of ourselves together, as the manner of some is; but exhorting one another: and so much the more, as ye see the day approaching.”

NOTE: For further reading have a look at: 
“No one misses out – 2024.05.30” https://missioncommunities.network/2024/05/30/no-one-misses-out-2024-05-30/
How to gain a friend – 2024.06.08 https://missioncommunities.network/2024/06/08/how-to-gain-a-friend-2024-06-08/
How to keep friends – 2024.06.09 https://missioncommunities.network/2024/06/09/how-to-keep-friends-2024-06-09/