The head coach named his 23-man group for this month’s competition in Czech Republic on Tuesday afternoon, after cutting down from an initial 27-strong long list announced last month.
And Southgate has revealed the thinking behind his decisions, with defender Eric Dier and forwards Patrick Bamford, Cauley Woodrow and Benik Afobe missing out.
UEFA European U21 Championship
The Finals
17-30 June 2015
Group A: Czech Republic, Denmark, Germany, Serbia
Group B: England, Italy, Portugal, Sweden
Prague, Uherske Hradiste & Olomouc, Czech Republic
"It’s been hard, but a couple of the guys always knew that they were already stand-by players," said Southgate after welcoming the squad to St. George’s Park.
"It’s a shame for Patrick, because he’s been injured and that’s why he misses out.
"We were conscious that towards the end of the season he wasn’t really fit to train with Middlesbrough, so he needs that period to get himself right now."
Tottenham defender Dier was perhaps the one player which caused Southgate the most thought, with plenty of competition for places in that position and Dier being young enough to feature for the U21s in the next Euro campaign.
"Eric was a difficult one," he admitted.
"That’s because we’ve got five very good centre-backs in that initial squad.
"Eric was last involved in the play-off game in Croatia and part of my thinking was that out of those five, Eric can be part of the U21s squad again next season.
"For the others, it’s their last opportunity so I’m very much looking at Eric as being one of our leaders next campaign and really taking ownership of the position on the field."
Wolves striker Afobe was named in one of Southgate’s squads for the first time last month, after a goal-laden campaign which saw him net 32 goals.
And Southgate admits he’s been impressed with the former Arsenal man after their conversations over the last few weeks.
"We felt we wanted to acknowledge the season that he’s had and it’s the same with Callum Wilson, who also wasn’t fit for the original squad," revealed Southgate.
"Benik had been in the England system at younger age groups and had a really good season last year and if we had any problems then I wouldn’t hesitate to call him in.
"He’s been really enthusiastic about potential involvement and I’ve been very impressed.
"But it’s always a difficult conversation to have because you’d like to give everybody the opportunity, but you’re not able to do that."
The majority of the squad have reported to St. George’s Park already, but for those players who have been involved with their clubs more recently, they will link-up for the second training camp next week.
"This first week is about getting the players back into training and we’ve given them all a period off after the end of their season," explained Southgate.
"We chose not to go abroad for our prep and I thought that was an important message really, so we gave the players some time last week and this week to take their own holidays.
"They’ll have little breaks between our camps, but we wanted to use St. George’s Park because it is home for us and there’s a good atmosphere in the group when we’re here.
"And everything is here for us, so when we work and we’re training it’s great to be able to use it."
England’s final send-off before they head out to the Euros takes place at Barnsley’s Oakwell on Thursday 11 June, when they face Belarus.
Tickets for England U21s v Belarus are on sale now, priced at £10 for adults and £5 for concessions. Call 0871 2266777 or e-mail boxoffice@barnsleyfc.co.uk for more details.