七 was originally a horizontal line crossed by a vertical cut — the picture of 'to cut'. It was borrowed for the sound 'seven', and the original cutting meaning moved to a new character, 切 (qiē, to cut). The little crossing cut is still visible.
Seven has mixed luck. It's tied to the Qixi Festival (七夕), the Chinese Valentine's Day, when the cowherd and weaver-girl stars meet once a year — so 七 can be romantic. But the seventh lunar month is the Ghost Month, when spirits roam, giving 7 a spookier side too.
A line with a cutting stroke through it — once 'cut', borrowed for SEVEN.
七夕 (the 'Seventh Eve') is China's Valentine's Day — the one night a year two star-crossed lovers reunite across the Milky Way. 七 carries that romance.