Let's dig deep into this.
1st gear has a ratio of 15.9934:1
2nd has a ratio of 10.9429:1
That's a change of 46%
3rd gear has a ratio of 8.7412:1. This is a 25% change from 2nd
4th has a ratio of 7.4924:1 - a 17% change from 3rd
5th has a ratio of 6.6897:1 - a 12% change from 4rd
6th has a ratio of 6.0876:1 - a 10% change from 5rd
The larger the change in ratio, the harder it is to be smooth. Moreover, 1st isn't intended to be a "riding" gear, it's meant to be a "get you going" gear. As you shift through the gears from 1st to 6th it gets easier and easier to be smooth. Conversely, when you begin to move back down to 1st you have to begin to really concentrate on your down shifts. Mentioned earlier by Trepetti, you need to blip the throttle (advance the throttle while the clutch is pulled in) to get a smooth clutch release and gear engagement. Throttle blipping is easy in gears 6 down to 4, and takes a little effort from 4 down through 2. But because 1st has such a tall gear (46% higher than 2nd), it takes an experienced set of hands to make it smooth.
The other issue with down-shifting to 1st at a high rpm - you run the risk of miscalculating and over reving your engine. If you float a valve, your engine will require expensive repairs.
Finally, you are riding on public roads. You need to be conscience & aware that you riding around at high rpm in low gears sounds frightening to others. And it really draws the attention of the police. Keeping a low profile will help you keep your license.