There are many ways to love; familial, romantic, erotic, a close friendship and selfless love. I believe that this sign is referring to the last one- selfless love. The Greeks had a word for it - "agape". Selfless love at its finest; the sort of love that God has for man. As far as that "neighbor thing"(true, selfless love) goes, Jesus had something to say about that!
Consider the following...
Luke, Chapter 19 (World English Bible)
25 Behold, a certain lawyer stood up and tested him, saying, “Teacher, what shall I do to inherit eternal life?”
26 He said to him, “What is written in the law? How do you read it?”
27 He answered, “You shall love (true, selfless love, or agape. GDR) the Lord your God with all your heart, with all your soul, with all your strength, and with all your mind; and your neighbor as yourself.”
28 He said to him, “You have answered correctly. Do this, and you will live.”
29 But he, desiring to justify himself, asked Jesus, “Who is my neighbor?”
30 Jesus answered, “A certain man was going down from Jerusalem to Jericho, and he fell among robbers, who both stripped him and beat him, and departed, leaving him half dead. 31 By chance a certain priest was going down that way. When he saw him, he passed by on the other side. 32 In the same way a Levite also, when he came to the place, and saw him, passed by on the other side. 33 But a certain Samaritan, as he traveled, came where he was. When he saw him, he was moved with compassion, 34 came to him, and bound up his wounds, pouring on oil and wine. He set him on his own animal, brought him to an inn, and took care of him. 35 On the next day, when he departed, he took out two denarii, gave them to the host, and said to him, ‘Take care of him. Whatever you spend beyond that, I will repay you when I return.’ 36 Now which of these three do you think seemed to be a neighbor to him who fell among the robbers?”
37 He said, “He who showed mercy on him.”
Then Jesus said to him, “Go and do likewise.”
26 He said to him, “What is written in the law? How do you read it?”
27 He answered, “You shall love (true, selfless love, or agape. GDR) the Lord your God with all your heart, with all your soul, with all your strength, and with all your mind; and your neighbor as yourself.”
28 He said to him, “You have answered correctly. Do this, and you will live.”
29 But he, desiring to justify himself, asked Jesus, “Who is my neighbor?”
30 Jesus answered, “A certain man was going down from Jerusalem to Jericho, and he fell among robbers, who both stripped him and beat him, and departed, leaving him half dead. 31 By chance a certain priest was going down that way. When he saw him, he passed by on the other side. 32 In the same way a Levite also, when he came to the place, and saw him, passed by on the other side. 33 But a certain Samaritan, as he traveled, came where he was. When he saw him, he was moved with compassion, 34 came to him, and bound up his wounds, pouring on oil and wine. He set him on his own animal, brought him to an inn, and took care of him. 35 On the next day, when he departed, he took out two denarii, gave them to the host, and said to him, ‘Take care of him. Whatever you spend beyond that, I will repay you when I return.’ 36 Now which of these three do you think seemed to be a neighbor to him who fell among the robbers?”
37 He said, “He who showed mercy on him.”
Then Jesus said to him, “Go and do likewise.”
Love for God is easy for me to understand, love for man is more difficult. Why? Because truly loving your fellow man often involves sacrifice and if the person is, say, less that socially acceptable (i.e. the homeless, a drug addict, a convict, sex-offender, member of a religious cult, habitual liar, a thief, etc.) most of us think twice before helping. In the parable of the "Good Samaritan", Jesus shows us that sometimes "religious" people may not exhibit their "love" for both God AND MAN. When a hated Samaritan does the good that they (religious people like a Priest and a Levite) should have done, it gives a lesson to all that anyone can love another human being.
In fact, loving God and loving man are linked together, for you can't truly love God without loving your fellow human being as well. The apostle John puts it this way...
1 John, Chapter 4 (WEB)
20 If a man says, “I love God,” and hates his brother, he is a liar; for he who doesn’t love his brother whom he has seen, how can he love God whom he has not seen? 21 This commandment we have from him, that he who loves God should also love his brother.
20 If a man says, “I love God,” and hates his brother, he is a liar; for he who doesn’t love his brother whom he has seen, how can he love God whom he has not seen? 21 This commandment we have from him, that he who loves God should also love his brother.
Somehow that sign takes on a new meaning, doesn't it?