(Why) is it wrong to lie?

Lying is wrong because it causes harm.

Lying deprives communication of its purpose, and makes it harder for individuals to trust each other and cooperate, when they don't know if the information they receive from each other is true.

Lying creates a situation in which the liar deprives another person of information they may need, and replaces it with false information that may drive them to make a wrong decision.

Lying, in this way, causes harm to the individual being lied to, and long term harm to the society that can no longer rely on communication as a way to convey true information and cooperate.

Because you steal people's power to make the best choices for themselves with having the all the information, and selfishly take their power only to benefit yourself. It's a blatant disregard for what you may put someone through with them believing in some lie you told. In short it's never good to do anything to anyone you do not want to be done to yourself and lies also has it's way causing harm to yourself. It's a rather frustrating, unnecessary, waste of time and energy.

One reason is that we become liars. Lying changes who we are. Our thoughts turn to how to keep the lie a secret and so we start to fabricate a story which we then have to believe is true. We will soon start to believe our own lies are the truth.

In the Bible, this is called giving false testimony, and it is the ninth of The Ten Commandments (Exodus 20:16).
The Hebrew rendered as 'false witness' (testimony) refers not just to falsifying the evidence, though that is included, but to speaking what is worthless, useless or unfounded. It can refer to speech that is empty of value, pregnant with dark innuendos, or a blatant lie.

Jesus also expanded on who our neighbour actually is - everyone who comes across our path - Luke 10:25-37 - for God requires us to love our neighbour. Included in that law is not telling lies to, or about our neighbour. Jesus also identified Satan the devil as "the father of lies" and said those who do likewise make him their 'father' - John 8:44-44. What more powerful reason could there be to avoid lying than that?!?

Exodus 23:1-3 details how that 9th commandment is to be worked out: "Do not spread false reports. Do not help a wicked man by being a malicious witness. Do not follow the crowd in doing wrong. When you give testimony in a lawsuit, do not pervert justice by siding with the crowd, and do not show favouritism to a poor man in his lawsuit."

The Christian writer James gave the most powerful denunciation of speaking wrongly: "The tongue also is a fire, a world of evil among the parts of the body. It corrupts the whole person, sets the whole course of his life on fire, and is itself set on fire by hell (James 3:6) This includes all gossip, malicious slander, a quarrelsome spirit, hasty words, coarse joking, flattery, the silent lie and unholy lies.

This latter category deals with false testimony with or about God by those who claim to speak for God. When they lie, they are false prophets. God never says the false prophets are simply mistaken, he says they are liars; “The prophets prophesy lies… my people love it this way” 2They are prophesying lies in my name” (Jeremiah 5:31 & 27:15). Well meaning or not, a prophetic lie was punishable by death (Jer. 14:13-16; 28:16-17 Zechariah 13:3). Spiritual lying comes under a far greater condemnation from God than any other kind of lying. In the same way those who profess to teach others will be judged more strictly (James 3:1). With God there is no colour coding for lies. White lies are like white magic, they are evil because they come from the same source who is the ‘father of lies’ (John 8:44).

There is much more on this, as detailed in the book below, so I encourage you to get that book, to plumb the depths of why God hates liars and will keep unrepentant liars out of His Kingdom (Revelation 21L8).

The opposite of truth. Lying generally involves saying something false to a person who is entitled to know the truth and doing so with the intent to deceive or to injure him or another person. A lie need not always be verbal. It can also be expressed in action, that is, a person may be living a lie. The Hebrew verb that conveys the idea of speaking that which is untrue is ka·zavʹ. (Pr 14:5) Another Hebrew verb sha·qarʹ means “deal or act falsely,” and the noun form is rendered “lie; deception; falsehood.” (Le 19:11; Ps 44:17; Le 19:12; Ps 33:17; Isa 57:4) Hebrew shawʼ, at times rendered “untruth; falsehood,” basically refers to something worthless, vain, valueless. (Ps 12:2; De 5:20; Ps 60:11; 89:47; Zec 10:2) The Hebrew verb ka·chashʹ (deceive) evidently has the basic meaning “prove disappointing.” (Le 19:11; Ho 9:2) The Greek term pseuʹdos and related words have to do with lying and falsehood.

The father, or originator, of lying is Satan the Devil. (Joh 8:44) His lie conveyed by means of a serpent to the first woman Eve ultimately brought death to her and to her husband Adam. (Ge 3:1-5, 16-19) That first lie was rooted in selfishness and wrong desire. It was designed to divert the love and obedience of the first human pair to the liar, who had presented himself as an angel of light, a benefactor. (Compare 2Co 11:14.) All other malicious lies uttered since that time have likewise been a reflection of selfishness and wrong desire. People have told lies to escape deserved punishment, to profit at the expense of others, and to gain or maintain certain advantages, material rewards, or the praise of men.

(Why), do you think it's wrong to lie? Wrong according to who?

Because all i have in this world is my balls and my word :)

It causes an element of distrust to emerge. Too much can undermine relationships.

God hates a liar. I for one would not want God to hate me.

First because God doesn't want us to lie, God likes the Truth:

"God, who cannot lie, promised before times long lasting" (Titus 1:3. Bible).

Second the practice will lead to destruction:

"Now we know that the Law is fine provided one handles it lawfully 9 in the knowledge of this fact, that law is promulgated, not for a righteous man, but for persons lawless and unruly, ungodly and sinners, lacking loving-kindness, and profane, murderers of fathers and murderers of mothers, manslayers, 10 fornicators, men who lie with males, kidnappers, liars, false swearers, and whatever other thing is in opposition to the healthful teaching 11 according to the glorious good news of the happy God, with which I was entrusted" (1 TImothy 1:8-11. Bible).