The gemara says in Taanit (29a):
מִשֶּׁנִּכְנַס אדר מַרְבִּין בְּשִׂמְחָה.
When adar comes in, we increase simcha.
What is an adar?
If we look at other places where the same word is used, we find this in Beitzah (15b):
וְאָמַר רַבִּי יוֹחָנָן מִשּׁוּם רַבִּי אֱלִיעֶזֶר בְּרַבִּי שִׁמְעוֹן: הָרוֹצֶה שֶׁיִּתְקַיְּימוּ נְכָסָיו — יִטַּע בָּהֶן אדר, שֶׁנֶּאֱמַר: ״אַדִּיר בַּמָּרוֹם ה׳״.
One who wants his properties to be preserved and protected from ruin should plant an eder tree among them, as it is stated: "The Lord on high is mighty [adir]"
Rashi there explains that "adar" is a prominent tree, which everybody knows is on your property, so your property will be safe from adverse possession (squatters) because everybody knows that you have an adar tree.
It can be assumed part of the prominence of the tree is its size and longevity. If that is the case, it clearly cannot be brought into the house.
So how can a person increase happiness in the house if it is impossible to move the tree?
Bonus image: