Who is really your NEIGHBOR ?

But "neighbourliness" is a two-way thing. The parable about the the good Samaritan was in answer to a Jew who asked Jesus who his neighbour was, following that same Jew's question assertion about loving one's neighbour as oneself. In the parable, Jesus is surely saying that the man who fell into the hands of robbers was a neighbour to the passers-by, even though he, the wounded man, was in no position to show mercy, be neighbourly, or do anything much apart from lying at the roadside.

I certainly do not take the parable to mean that we are only to be merciful to those who show us mercy.

Think of the Lord Jesus Christ. Does He show mercy only to those who show Him mercy? Of course not. He died for sinners, that is, for those who were opposed to Him. Romans 5.6-8:

6 ¶ For when we were still without strength, in due time Christ died for the ungodly.
7 For scarcely for a righteous man will one die; yet perhaps for a good man someone would even dare to die.
8 But God demonstrates His own love toward us, in that while we were still sinners, Christ died for us. ​

Jesus said in Luke 6.31-36:

31 "And just as you want men to do to you, you also do to them likewise.
32 "But if you love those who love you, what credit is that to you? For even sinners love those who love them.
33 "And if you do good to those who do good to you, what credit is that to you? For even sinners do the same.
34 "And if you lend to those from whom you hope to receive back, what credit is that to you? For even sinners lend to sinners to receive as much back.
35 "But love your enemies, do good, and lend, hoping for nothing in return; and your reward will be great, and you will be sons of the Most High. For He is kind to the unthankful and evil.
36 "Therefore be merciful, just as your Father also is merciful."​

That would seem to go against any idea that Christians should only love those who show mercy.