Ok, so it's a bit longer story.

Basically, for quite a long time, the principalities of Wallachia and Moldavia were both under occupation of one or other of the 3 major empires bordering them: The Austro-Hungarian Empire, the Ottoman Empire and the Russian Empire.

Now, today's country Moldavia is actually about half of the former historical Molodavia, basically the part between the rivers Prut and Nistru.

In the 19th century the Ottoman Empire and the Russian Empire went to war again. At that time the two romanian provinces were subjects of the Ottoman Empire. The Russian Empire was winning quite good, but it got attacked by Napoleon. Not wanting to fight a war on two sides, they pushed for peace with the Ottoman Empire.

To make sure that the Ottomans don't refuse, instead of requesting the entire Moldova, they only requested half of it, the part between the Prut and Nistru rivers. The ottomans agreed, and said part, called Bassarabia, was incorporated into the Russian Empire.

Later on, in 1859, Moldova and Wallacha united, becoming Romania, but the more complete Romania only appeared after WW1. In WW1, Romania fought against the Central powers.

You might wonder, what does Bassarabia have to do with this, in the end Russia fought against the Central powers too. Well, Russia had their civil war at the time and different nations that had been under their control for some time profited to affirm their independence. Finland, Ukraine, Estonia, Lithuania, Latvia etc.

Among these was also Bassarabia, who also decided to unite with Romania as they were both romanian speaking and had a common past, but also due to the threat from Ukraine, who was feared would conquer the region.

So, after WW1, Grand Romania was born, encompassing former Wallachia, Moldova(including Bassarabia), Transylvania(former part of Austro-Hungary), Bukovina(former part of Austro-Hungary), Banat(former part of Austro-Hungary), Crisana(former part of Austro-Hungary) and Maramures(former part of Austro-Hungary).

However, after the communists started consolidating their power, they also started trying to regain regions lost. Ukraine was first, and, as you know, the Baltic states were annexed before WW2 as well, Finland lost a chunk and Romania was given an ultimatum, give back Bassarabia and Northern Bukovina(as penalty for having taken Basserabia from them) or face war.

Meanwhile Romania, who had tried to remain neutral after WW1, was facing another similar ultimatum from Hungary regarding a part of Transylvania (Hungary was already allied with Nazi Germany) and Bulgaria regarding a part of Dobruja (although that part was mostly theirs, taken just before WW1, but that's another story)

Romania decided to give in to these demands as it could not fight a war on 3 fronts. The king abdicated. When the Axis prepared their "operation Barbarossa" to invade USSR, Romania joined the Axis to regain at least part of its lost territoriy, Basserabia. It did, and even went into the region beyond it and then all the way to Stalingrad.

When romanians reentered Basserabia in 1940 they found out that many romanian families had disappeared, it was the USSR policy to transmute populations or get rid of them.

But eventually, the Axis lost. USSR took back Basserabia and Northern Bukovina and started a cleansing. It brought back ethnic russian people there and took many romanians and sent them to gulags in Siberia. Many never returned.

When USSR dismantled, the people living in Basserabia gained independence and named their country Moldova. The southern side of former Basserabia was no longer truly populated with Romanians, it's mostly russians and ukranians, so it did not become part of Moldova, and neither did Northern Bukovina as the independence movement was not strong enough there(even if there are still many romanians there).

Romania was not able to assist Moldova at this time as Romania was facing its anti-communist revolution and aftermath.

Afterwards Moldova became independent, but having lived in USSR for so long, many longed for the "old times" when "it was better". Basically nostalgia and part brainwashing. Considering that USSR had also promoted "moldavian" history as separate from that of romanians to ensure the people don't want unity and many people living there were not russians and ukrainians, the wish for unity was not that big anymore.

Romania had a policy to give special education permits to Moldavians hoping they would then go home and promote unity or something. Most however took that, applied for romanian citizenship then went to the EU when we joined.

Today, Romania is one of the poorest country in the EU. Mostly due to corruption that remained from communist times. Unfortunately, Moldova is even poorer, more corrupt, and it also has problems with Transnistria (which is an area almost fully occupied by russians between Moldova and Ukraine, but it is itself technically part of Moldova).

Basically, Romania could not afford to help Moldova that much. Germany could back at the reunification, being quite a rich country. Moldavians kind of know that, so the wish for unification isn't that huge there too. Basically, we are alike, but many romanians don't really want a country even poorer than them to be part of Romania and moldavians know there is no true benefit to be part of Romania as those that had an actual benefit (students) had received it long ago. People in both countries try to deal with struggling out of poverty and dealing with everyday life, and without the people wanting unity, there has never been a true action like a referendum or something to see if unity is desired anymore.