Arrogance will always despise whatever does not support its cause.  Jesus had previously used a small child to challenge the arrogance of the disciples who wanted prominence and precedence in His kingdom (Matthew 18:1-5). The child was as precious to Him as the disciples, but they could not see it.  They thought of themselves as being close to Jesus, but believed that the child was unimportant to Him.  After all, they worked for Jesus, but the child did not; they were known as being Jesus' special team, unlike the child who nobody knew.
 
This warning against arrogance was directed straight at them.  They thought that 'little ones' were inferior to themselves (they thought they were 'big ones') but Jesus said they were wrong.  Their assessment of themselves was not right because everybody must become a 'little one' if they are to enter Christ's kingdom (Matthew 18:3).  To assume greatness, and despise any of His people, is to despise Him (Matthew 25:34-40).
 
The reference to 'their angels in heaven' has been interpreted as meaning that every child has a ‘guardian angel’.  However attractive this might be, such a doctrine is not supported elsewhere in Scripture.  Hebrews 1:14 says that angels are sent by God to serve Christians, but they are under God’s command, and we do not know in what way they affect our lives.  Indeed, we are forbidden to worship angels, but commanded to trust in Jesus (Colossians 2:18; Revelation 22:8-9).  Jesus is making the point that God, and all those who adore Him, do not ignore or belittle anyone who comes to Him …  however lost and small they appear to be.  Therefore, His disciples must not despise other people (James 2:1-4).
 
It is wonderful to know that God loves us, but it is essential to know He does not love us any more than anyone else, because God is love (1 John 4:8).  He loves everybody (John 3:16), and longs that all will repent and be saved … whoever they are (1 Timothy 2:4).  To despise anybody who God loves, reveals that we do not share His heart of love.  If you are withholding your love from anyone, please be aware that God does not.  If you have no desire to stop discriminating against people, then ask yourself if you have become like a 'little one'.  If not, then humble yourself before it is too late.