There are three kinds of happiness: Aihikam of happiness in this life, Amushmikam of happiness in Swarga, and Nihshreyasam or happiness in Moksha. Aihikam is both mixed with sorrow and temporary, Amushmikam is unmixed with sorrow but still temporary, and Nihshreyasam is both unmixed with sorrow and eternal. For more detail, here is what Ishwara Krishna's Samkhya Karika says:
dṛṣṭavadānuśravikaḥ sa hyaviśuddhi kṣayātiśayayuktaḥ ।
tadviparītaḥ śreyān vyaktāvyaktajñavijñānāt ॥
The [means of happiness revealed by the Vedas] is like the evident one, It is linked with impurity, destruction and inequality. Other than that is better,—proceeding from the right cognition of the Manifest, Unmanifest and the Knower.
Here is how Vachaspati Mishra explains "destruction and inequality" in his Samkhya Karika Bhashya:
Though the terms decay and excess (used in the above Kārikā) really relate to the effect, here they are attributed to the means. This quality of decay in heaven is inferred as it is a positive entity and a product. Further, it is said that sacrifices like Jyotiṣṭoma are the means of attaining mere heaven, whereas sacrifices like Vājapeya etc. lead one to self-sovereignty. This inequality in the result is what constitutes excess spoken of (in the Kārikā). Verily, the superior prosperity of one man makes another of lesser prosperity sad!
Immortality denoted in the passage ‘We drank soma and became immortal’ indicates long durability. It is said elsewhere: ‘Verily, immortality is the durability extending till the final dissolution of all the elements (i.e. of the entire universe).’ Hence, the Śruti declares: ‘Neither by deeds nor by progeny nor by wealth but by renunciation alone they attained immortality; that which the hermits enter is laid beyond the heavens and yet it shines brilliantly in the heart’ (M.N.Up.12-14); and also, ‘Sages with children and desiring wealth got only death (as reward) by actions while those other sages who were wise attained immortality which is beyond all actions.’
So destruction refers to fact that residence in Swarga is not eternal; after you've exhausted the fruit of your actions that brought you there, you have to be reborn again. And inequality refers to the fact that not everyone in Swarga is of equal status. Doing an Ashwamedha Yagna only makes you a resident of Swarga, whereas doing a Vajapeya Yagna makes you Indra, i.e. the ruler of Swarga. So while residing in Swarga, you'll be jealous of others who have higher status than you do.
And also, happiness in Swarga is not as great as happiness in Moksha. This chapter of the Brihadaranyala Upanishad analyzes this quantitatively:
If a man is healthy, wealthy, and lord of others, surrounded by all human enjoyments, that is the highest blessing of men. Now a hundred of these human blessings make one blessing of the fathers who have conquered the world (of the fathers). A hundred blessings of the fathers who have conquered this world make one blessing in the Gandharva world. A hundred blessings in the Gandharva world make one blessing of the Devas by merit (work, sacrifice), who obtain their godhead by merit. A hundred blessings of the Devas by merit make one blessing of the Devas by birth, also (of) a Srotriya who is without sin, and not overcome by desire. A hundred blessings of the Devas by birth make one blessing in the world of Pragâpati, also (of) a Srotriya who is without sin, and not overcome. by desire. A hundred blessings in the world of Pragâpati make one blessing in the world of Brahman, also (of) a Srotriya who is without sin, and not overcome by desire. And this is the highest blessing.
So happiness in Swarga is either 10,000 or a million times less than the highest possible happiness.
To answer your side question, it is perfectly possible for a soul to be happy in Swarga without the presence of the spouse it had in its most recent birth.