Short answer. Generally no. Your private reason for signing is your motive, and the law treats motive as something different from the cause of the contract. A contract stands or falls on its cause, so a personal expectation that was never part of the bargain usually gives you no ground to attack the agreement.
What the law says
The particular motives of the parties in entering into a contract are different from the cause thereof.
Civil Code, Article 1351 — Cause vs. Motive. Read the full provision →
The cause is what the law examines
Article 1351 of the Civil Code settles the point in one sentence: The particular motives of the parties in entering into a contract are different from the cause thereof. The cause is the objective reason the law attaches to that kind of agreement — in a sale, the price for the seller and the thing for the buyer; in a loan, the money actually delivered. It is the same for everyone who enters that type of contract, which is why a court can test it. A contract with no cause at all, or with an unlawful one, does not produce the effect the parties intended.
Your motive is your own affair
The motive is the private, particular reason that pushed you personally to sign: you bought the lot because you expected a road to be built beside it, or you sold the truck because you thought a relative would repay you later. Those reasons sit in your head, not in the agreement. The other party did not bargain for them and cannot be made to carry the risk that they failed. So the general rule is unforgiving: a motive that is disappointed, mistaken or simply never realised does not by itself make the contract defective, and it is not a defence when the other side sues to enforce it.
When a private reason stops being merely a motive
The separation is not absolute. A reason ceases to be a bare motive once the parties themselves build it into the agreement — by writing it in as a condition, by making performance depend on it, or by making it so plainly the determining reason for contracting, and known to both sides, that it merges into the cause itself. Where that happens, a motive that is false or contrary to law can taint the contract, because what is being attacked is no longer a private hope but the very reason the agreement exists. That is a demanding thing to establish, and it turns on what the parties said and did, not on what one of them privately felt.
What this means when you want out
If you are trying to escape a contract, regret about why you signed is the weakest position to argue from. Look instead at whether there was a real cause, whether it was lawful, and whether your consent was obtained by mistake, violence, intimidation, undue influence or fraud — those attack the agreement on grounds the law recognises. Note too that actions to annul are subject to time limits, and continuing to perform after you learn of a defect can be treated as accepting the contract. Keep the drafts, messages and receipts that show what was actually agreed, and take advice on your specific documents before you stop performing.